JP2008076191A - Failure diagnosis device of oxygen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To determine discriminatingly determine a failure of an oxygen sensor and a circuit failure. <P>SOLUTION: This failure diagnosis device of an oxygen sensor generating an electromotive force corresponding to an oxygen concentration in exhaust gas is equipped with an element impedance detection circuit for detecting an element impedance of the oxygen sensor in a process for applying a voltage to the oxygen sensor and drawing it back, a sensor output voltage detection circuit for detecting an output voltage from the oxygen sensor, and an impedance reference failure determination means for discriminatingly determining a failure of the oxygen sensor and a failure of the element impedance detection circuit corresponding to at least the detected element impedance value when a voltage difference ΔVOX of the detected oxygen sensor output voltage before and after application is larger than a prescribed value. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an oxygen sensor failure diagnosis device, and more particularly to an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas.

  In an internal combustion engine equipped with an exhaust gas purification system using a catalyst, in order to effectively remove harmful components of exhaust gas by the catalyst, the mixture ratio of air and fuel in the air-fuel mixture burned in the internal combustion engine, that is, the air-fuel ratio is reduced. Control is essential. In order to perform such air-fuel ratio control, an oxygen sensor for detecting the oxygen concentration of the exhaust gas is provided in the exhaust passage of the internal combustion engine, the air-fuel ratio is obtained from the detection result, and the detected air-fuel ratio is set to a predetermined target air-fuel ratio. The feedback control to bring it closer to

  The oxygen sensor includes a cylindrical detection element disposed so as to protrude into the exhaust passage. The detection element has its inner surface exposed to the atmosphere (air), and its outer surface is exposed to exhaust gas flowing through the sensor cover. The detection element is formed of a solid electrolyte having inner and outer surfaces covered with electrodes. The solid electrolyte refers to a solid substance that can move in the state in which oxygen is ionized. For example, zirconia is used as an oxygen sensor. If there is a difference in oxygen partial pressure between the atmosphere inside the sensing element and the exhaust gas outside, oxygen on the higher oxygen partial pressure (usually the atmosphere side) is ionized to reduce the difference in partial pressure. It moves through the solid electrolyte to the low oxygen partial pressure side (usually the exhaust gas side). Oxygen molecules receive tetravalent electrons in the process of ionization, and emit tetravalent electrons in the process of returning from the ionized state to the molecule. Therefore, movement of electrons occurs at the electrodes on the inner and outer surfaces of the detection element in accordance with the movement of oxygen, and as a result, an electromotive force is generated in the detection element. Thus, the oxygen sensor generates an electromotive force according to the difference in oxygen partial pressure between the atmosphere and the exhaust gas. More specifically, the oxygen concentration of the exhaust gas decreases (that is, the air-fuel ratio of the exhaust gas becomes richer). It generates a large electromotive force.

  Incidentally, the electromotive force generated by the oxygen sensor is sent as an output voltage to the electronic control unit. On the other hand, when the electronic control unit detects an abnormal output voltage, it is determined that a failure has occurred at any location. The identification of the faulty part is important for prompt and accurate repair work such as later part replacement. As a failure diagnosis apparatus capable of specifying such a failure part, for example, there is a device that specifies whether a short circuit has occurred outside the port of the control unit or a switch element in the control unit is faulty ( Patent Document 1).

  In addition, as a conventional failure diagnosis apparatus, the ratio of the number of times the inversion is performed with an increase in the intake air amount is predetermined with respect to the total number of times that the output signal of the oxygen sensor is inverted from the rich signal to the lean signal. There is one that determines that a fine breakage has occurred in the detection element of the oxygen sensor based on the fact that it is equal to or greater than the value (see Patent Document 2). Also, the output voltage of the oxygen sensor is monitored, and it is determined that the detection element of the oxygen sensor is missing when the output distribution in which the output ratio of the lean signal is equal to or greater than a predetermined value is confirmed, and abnormality diagnosis of the sensor is performed. (See Patent Document 3).

JP 2005-140642 A JP 2004-346847 A JP 2003-14683 A

  By the way, as described in Patent Documents 2 and 3, in the oxygen sensor, there may be a deficiency failure in which the detection element is cracked or the detection element is cracked. When this defect occurs, the inside and outside of the detection element communicate with each other, and exhaust gas enters and exits the inside of the detection element that originally contains air. The sensor output voltage fluctuates due to the entry and exit of the exhaust gas. Therefore, a failure of the oxygen sensor can be detected by monitoring the fluctuation of the sensor output voltage.

  However, even when a failure occurs in an electric circuit portion other than the oxygen sensor, a signal with fluctuations may be obtained by the electronic control unit as in the case of the oxygen sensor failure. In this case, there is a problem in the electronic control unit that it is impossible to determine whether the oxygen sensor is faulty or the circuit is faulty, and the faulty part cannot be accurately identified.

  Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide an oxygen sensor failure diagnosis apparatus capable of distinguishing and determining an oxygen sensor failure and a circuit failure.

In order to achieve the above object, the first invention provides:
In an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas,
Applying a predetermined element impedance detection voltage to the oxygen sensor and then pulling it back, and an element impedance detection circuit for detecting the element impedance of the oxygen sensor in this application process;
A sensor output voltage detection circuit for detecting an output voltage of the oxygen sensor;
When the voltage difference of the oxygen sensor output voltage before and after the application detected by the sensor output voltage detection circuit or the absolute value thereof is larger than a predetermined value, at least according to the value of the element impedance detected by the element impedance detection circuit, Impedance reference failure determination means for distinguishing and determining a failure of the oxygen sensor and a failure of the element impedance detection circuit.

  In either case of a sensor failure or a circuit failure, the oxygen sensor output voltage before and after the application is different, and the voltage difference or the absolute value thereof may be larger than a predetermined value. However, the oxygen sensor cannot generate an output voltage if the detection element temperature of the oxygen sensor is a low temperature lower than the activation temperature. The value of the element impedance of the oxygen sensor is a value correlated with the detection element temperature. In view of these viewpoints, in the first invention, the sensor failure and the circuit failure are distinguished and determined according to the value of the element impedance. As a result, it is possible to specify a failure site, and to perform a failure diagnosis with high accuracy.

The second invention is the first invention, wherein
The impedance reference failure determination means determines that the device impedance detection circuit has failed when the detected value of the device impedance is greater than a predetermined value.

The third invention is the first or second invention, wherein
The impedance reference failure determination means determines that the oxygen sensor has failed when the value of the detected element impedance is smaller than a predetermined value.

In addition, the fourth invention is
In an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas,
Applying a predetermined element impedance detection voltage to the oxygen sensor and then pulling it back, and an element impedance detection circuit for detecting the element impedance of the oxygen sensor in this application process;
A sensor output voltage detection circuit for detecting an output voltage of the oxygen sensor;
Intake air amount detection means for detecting the intake air amount of the internal combustion engine;
When the value of the element impedance detected by the element impedance detection circuit is smaller than a predetermined value, a failure of the oxygen sensor according to at least the value of the intake air amount detected by the intake air amount detection means, and the element An air amount reference failure determination means for distinguishing and determining a failure of the impedance detection circuit.

  When the value of the element impedance of the oxygen sensor is smaller than a predetermined value (for example, when the detection element temperature is at the activation temperature), if the oxygen sensor is in a failure state, its output voltage varies depending on the value of the intake air amount. Or not. Therefore, using this characteristic, in the fourth aspect of the invention, the sensor failure and the circuit failure are distinguished and determined according to the value of the intake air amount. This also makes it possible to specify a faulty part and to perform fault diagnosis with high accuracy.

The fifth invention is the fourth invention, wherein
A variable integrated amount calculating means for calculating a variable integrated amount of the output voltage of the oxygen sensor;
The air amount reference failure determining means has a medium value of the detected intake air amount, and when the value of the fluctuation integrated amount calculated by the fluctuation integrated amount calculating means is larger than a predetermined value, It is determined that the oxygen sensor has failed.

Moreover, 6th invention is 4th or 5th invention,
The air amount reference failure determination means has a value of the detected intake air amount being a low or high value, and the voltage of the oxygen sensor output voltage before and after the application detected by the sensor output voltage detection circuit. When the difference or the absolute value thereof is greater than a predetermined value, it is determined that the element impedance detection circuit has failed.

In addition, the seventh invention,
In an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas,
Applying a predetermined element impedance detection voltage to the oxygen sensor and then pulling it back, and an element impedance detection circuit for detecting the element impedance of the oxygen sensor in this application process;
A sensor output voltage detection circuit for detecting an output voltage of the oxygen sensor;
Intake air amount detection means for detecting the intake air amount of the internal combustion engine;
Fluctuation integration amount calculation means for calculating a fluctuation integration amount of the detected output voltage of the oxygen sensor;
At least one of the value of the element impedance detected by the element impedance detection circuit and the value of the intake air amount detected by the intake air amount detection means, and the oxygen sensor before and after the application detected by the sensor output voltage detection circuit Based on the voltage difference of the output voltage or the absolute value thereof and at least one of the values of the fluctuation integration amount calculated by the fluctuation integration amount calculation means, the failure of the oxygen sensor is distinguished from the failure of the element impedance detection circuit. And an impedance / air quantity reference failure judging means for judging.

  In the seventh aspect of the present invention, preferably, the impedance / air amount reference failure determination means has a value of the element impedance detected by the element impedance detection circuit larger than a predetermined value and is detected by the sensor output voltage detection circuit. Further, when the voltage difference of the oxygen sensor output voltage before and after the application or the absolute value thereof is larger than a predetermined value, it is determined that the element impedance detection circuit has failed.

  In the seventh invention, it is preferable that the impedance / air amount reference failure determination means has an element impedance value detected by the element impedance detection circuit smaller than a predetermined value, and the intake air amount detected by the intake air amount detection means. When the value of the air amount is a low or high value, and the voltage difference or absolute value of the oxygen sensor output voltage before and after the application detected by the sensor output voltage detection circuit is larger than a predetermined value, the element It is determined that the impedance detection circuit is faulty.

  In the seventh invention, it is preferable that the impedance / air amount reference failure determination means has an element impedance value detected by the element impedance detection circuit smaller than a predetermined value, and the intake air amount detected by the intake air amount detection means. When the value of the air amount is a medium value and the value of the variation integrated amount calculated by the variation integrated amount calculating means is larger than a predetermined value, it is determined that the oxygen sensor has failed.

Further, an eighth invention is any one of the first to seventh inventions,
The failure of the element impedance detection circuit is a failure of a switch element that is turned on when the voltage is pulled back from the oxygen sensor.

  According to the present invention, it is possible to distinguish between a failure of an oxygen sensor and a circuit failure and determine the failure part, and it is possible to identify a failure part and perform a highly accurate failure diagnosis.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  A configuration of an exhaust gas purification system for an in-vehicle internal combustion engine to which the present invention is applied will be described with reference to FIG. The intake passage 11 of the internal combustion engine 10 is provided with a throttle valve 15 (in this embodiment, electronically controlled) whose variable passage area is provided, and the amount of air taken in through the air cleaner 14 is adjusted by opening degree control. The The amount of air sucked here (intake air amount) is detected by the air flow meter 16. The air sucked into the intake passage 11 is mixed with fuel injected from an injector 17 provided downstream of the throttle valve 15 and then sent to the combustion chamber 12 where it is burned.

  On the other hand, a three-way catalyst 18 for purifying harmful components in the exhaust gas is provided in the exhaust passage 13 through which the exhaust gas generated by the combustion in the combustion chamber 12 is sent. A post-catalyst oxygen sensor 20 is provided on each downstream side.

  The three-way catalyst 18 efficiently purifies all the main harmful components (HC, CO, NOx) in the exhaust gas only when the air-fuel ratio of the air-fuel mixture to be burned is in a narrow range (window) near the theoretical air-fuel ratio. . In order for such a three-way catalyst 18 to function effectively, it is necessary to strictly control the air-fuel ratio of the air-fuel mixture to match the center of the window.

  Such air-fuel ratio control is performed by an electronic control unit (hereinafter referred to as “ECU”) 22. The ECU 22 receives detection signals from various sensors such as the air flow meter 16, the oxygen sensors 19, 20 or the accelerator sensor 21 that detects the amount of depression of the accelerator pedal, and the NE sensor 23 that detects the engine speed. ing. The throttle valve 15 and the injector 17 are driven and controlled in accordance with the operating conditions of the internal combustion engine 10 and the vehicle ascertained from the detection signals of the sensors, thereby controlling the air-fuel ratio as described above. The outline of the air-fuel ratio control by the electronic control unit 22 is as follows.

  First, the electronic control unit 22 obtains a required amount of intake air amount that is grasped according to the amount of depression of the accelerator pedal and the detection result of the engine speed, and the throttle valve 15 of the throttle valve 15 is obtained so as to obtain the intake air amount accordingly. Adjust the opening. On the other hand, an amount of fuel sufficient to obtain the stoichiometric air-fuel ratio is obtained from the actually measured value of the intake air amount detected by the air flow meter 16, thereby adjusting the fuel injection amount from the injector 17. Thereby, the air-fuel ratio of the air-fuel mixture burned in the combustion chamber 12 can be brought close to the stoichiometric air-fuel ratio to some extent. However, that alone is not sufficient for the required highly accurate air-fuel ratio control.

  Therefore, the electronic control unit 22 feedback corrects the fuel injection amount from the injector 17 based on the actual measured value of the air-fuel ratio obtained from the detection results of the oxygen sensors 19 and 20, and the required accuracy of the air-fuel ratio control. Is secured.

  As described above, in this exhaust gas purification system, the so-called air-fuel ratio feedback control that performs feedback correction of the fuel injection amount according to the detection results of the oxygen sensors 19 and 20 is performed, so that the air-fuel ratio of the air-fuel mixture is reduced to the theoretical air-fuel ratio. A high exhaust gas purification rate is secured by maintaining the fuel ratio in the vicinity. In this exhaust gas purification system, as described above, the oxygen partial pressures of the exhaust gas upstream and downstream of the three-way catalyst 18 are detected by the two oxygen sensors 19 and 20, respectively. High accuracy is achieved.

  The two oxygen sensors 19 and 20 employed in such an exhaust purification system have the same configuration as each other, and the failure diagnosis method is also the same. Therefore, the pre-catalyst oxygen sensor 19 will be described as an example, and the post-catalyst oxygen sensor 20 will not be described. As shown in FIGS. 2 and 3, the oxygen sensor 19 includes a cylindrical detection element (sensor element) 31 disposed so as to protrude into the exhaust passage 13. The detection element 31 has its inner surface exposed to the atmosphere (air), and its outer surface is exposed to exhaust gas flowing through the sensor cover 32. The detection element 31 is formed of a solid electrolyte in which electrodes 33A and 33B are coated on the inner and outer surfaces. The solid electrolyte refers to a solid substance that can move in the state in which oxygen is ionized. For example, zirconia is used as an oxygen sensor. The atmospheric chamber 34 inside the detection element 31 communicates with the outside through an atmospheric passage (not shown) provided in the sensor and an atmospheric hole 35 formed in the sensor body, and the atmospheric air is introduced. . The atmosphere chamber 34 is provided with a heater 36 for heating the detection element 31 and activating it at an early stage. The heater 36 is energized and controlled by the ECU 22.

  When a difference occurs in the oxygen partial pressure between the inner atmosphere separated from the detection element 31 and the outer exhaust gas, in order to reduce the difference in the partial pressure, the oxygen partial pressure side (usually the atmosphere side) is reduced. ) Is ionized, passes through the solid electrolyte, and moves to the side where oxygen partial pressure is low (usually the exhaust gas side). Oxygen molecules receive tetravalent electrons in the process of ionization, and emit tetravalent electrons in the process of returning from the ionized state to the molecule. Therefore, movement of electrons occurs at the electrodes on the inner and outer surfaces of the detection element 31 in accordance with the movement of oxygen, and as a result, an electromotive force is generated in the detection element 31. Thus, the oxygen sensor 19 generates an electromotive force according to the difference in oxygen partial pressure between the atmosphere and the exhaust gas. More specifically, as the oxygen concentration of the exhaust gas decreases (that is, the exhaust gas outside the detection element 31). Larger electromotive force is generated (the richer the air-fuel ratio). Here, since oxygen ions are directed from the inner surface side electrode 33A through the detection element 31 to the outer surface side electrode 33B, the direction of the current is reversed, and the inner surface is not connected to the external device connected to both electrodes. The side electrode 33A is a positive electrode, and the outer surface side electrode 33B is a negative electrode.

  Incidentally, there are various types of oxygen sensors such as those using a plate-shaped detection element and those using a material other than zirconia for the detection element. In many cases, a configuration for detecting the oxygen partial pressure of the exhaust gas based on the same detection principle as that of the above-described sensor, that is, a detection element arranged to isolate the reference gas (atmosphere) and the exhaust gas is a reference gas. The electromotive force is generated in accordance with the difference in oxygen partial pressure of the exhaust gas with respect to the exhaust gas.

  The output characteristics of the oxygen sensor 19 are illustrated in FIG. As shown, the output voltage of the oxygen sensor 19 changes transiently at the theoretical air-fuel ratio A / Fs (for example, 14.6), and the air-fuel ratio A of the exhaust gas (atmosphere gas) supplied to the oxygen sensor 19 In a region where / F is leaner than the stoichiometric air-fuel ratio A / Fs (A / F> A / Fs, hereinafter also referred to as lean air-fuel ratio), it shows a small voltage of about 0.1 V, richer than the stoichiometric air-fuel ratio A / Fs In such a region (A / F <A / Fs, hereinafter also referred to as a rich air-fuel ratio), a relatively high voltage of about 0.9 V is shown. Here, the sensor output of 0.45 V is used as a rich / lean determination threshold value to determine whether the detection result of the sensor 19 is richer or leaner than the stoichiometric air-fuel ratio. Note that the magnitude of the sensor output voltage in each region of the oxygen sensor 19 may vary depending on the temperature state of the detection element 31.

  Note that, as in this embodiment, in an internal combustion engine that performs air-fuel ratio control for the purpose of combustion only at the stoichiometric air-fuel ratio (stoichiometric combustion), an oxygen sensor having a characteristic that the output voltage changes greatly with the stoichiometric air-fuel ratio as a boundary. Often used. Such sensors have a lower resolution, either richer than the stoichiometric air-fuel ratio and leaner than the stoichiometric air-fuel ratio, but are often sufficient to perform only the stoichiometric combustion. On the other hand, in an internal combustion engine that performs combustion at a wider range of air-fuel ratio, such as combustion at a lean air-fuel ratio, the output voltage varies linearly with the air-fuel ratio of exhaust gas, and the resolution is higher. An oxygen sensor may be used. The present invention is also applicable to such an oxygen sensor.

  By the way, due to aged deterioration due to long-term use or the like, the detection element 31 of the oxygen sensor 19 may be cracked or the detection element 31 may be broken, and the oxygen sensor 19 may break down. In the case of a sensor failure due to this defect, as shown in FIG. 5, the inside and outside of the detection element 31 communicate with each other through the defect part 37 of the detection element 31, and exhaust gas outside the detection element 31 enters and exits the inside. When exhaust gas exists inside and outside the detection element 31 and there is no difference in the oxygen partial pressure between the inside and outside, no sensor electromotive force is generated, and the exhaust gas is contained inside the detection element 31. When exhaust gas having a high oxygen concentration (air-fuel ratio lean) exists outside the detection element 31, an electromotive force in the reverse direction is generated in the oxygen sensor 19. This can occur, for example, immediately after the air-fuel ratio is switched from rich to lean in a sensor failure state or immediately after a fuel cut. In this case, the potential of the negative electrode 33B is higher than the potential of the positive electrode 33A, and a negative (minus) electromotive force is generated. Due to this principle, when the oxygen sensor 19 has a deficiency failure, the air-fuel ratio of the exhaust gas cannot be accurately detected, and the output voltage value of the oxygen sensor 19 fluctuates in small increments due to the exhaust gas entering and exiting the detection element 31. However, noise components are superimposed.

  This deficiency failure of the oxygen sensor 19 can be detected by the ECU 22 by the following method. As shown in FIG. 6, at each processing timing T (n) (n = 1, 2, 3,...) Separated by a predetermined period, the oxygen sensor output voltage Vs (n) acquired at the timing and 1 from it. The absolute value | Vs (n) −Vs (n−1) | of the difference from the oxygen sensor output voltage Vs (n−1) obtained at the previous timing is calculated, and this value is calculated for each processing timing T (n ) Accumulated sequentially every time. When the value obtained by integrating for a predetermined time in this way, that is, the fluctuation integrated amount (or integrated locus length) is larger than the predetermined value, the oxygen sensor 19 can be determined to be faulty.

  However, as described above, it has been confirmed that even when a failure occurs in an electric circuit portion other than the oxygen sensor 19, a signal with fluctuations may be obtained by the ECU 22 in the same manner. Therefore, in such a case, the ECU 22 cannot determine whether the fluctuation is caused by a failure of the oxygen sensor or a failure of the circuit, and the failure part can be accurately identified by the above method. Can not. If the failure part cannot be specified, it is inevitably known which part should be replaced, and there is a disadvantage that it is not possible to quickly cope with the failure.

  Therefore, in the present embodiment, the determination is made by distinguishing between the failure of the oxygen sensor and the circuit failure as follows. Since the method for determining the failure is the same for both oxygen sensors 19 and 20, only one of the pre-catalyst oxygen sensors 19 will be described here as an example.

  First, a circuit configuration including the oxygen sensor 19 and the ECU 22 to which the oxygen sensor 19 is connected will be described with reference to FIG. As shown in the figure, the oxygen sensor 19 has its positive electrode 33A connected to the ECU 22 by a positive electrode wire 41A and its negative electrode 33B by a negative electrode wire 41B. Specifically, the positive electrode line 41 </ b> A and the negative electrode line 41 </ b> B are connected to a positive electrode terminal 42 </ b> A and a negative electrode terminal 42 </ b> B provided in a casing (indicated by a thick solid line) of the ECU 22, respectively. The oxygen sensor 19 is equivalently shown as including an impedance component Rs and an electromotive force component. The ECU 22 includes a central processing unit (hereinafter referred to as CPU) 43, and the CPU 43 is connected to the positive terminal 42A and the negative terminal 42B through the following circuit.

  The ECU 22 has a function of acquiring information related to the exhaust air-fuel ratio based on a voltage generated by the oxygen sensor 19 (voltage between the positive electrode 33A and the negative electrode 33B) and a function of detecting the element impedance Rs of the oxygen sensor 19. Among these, the part that exhibits the function of detecting the element impedance Rs of the detection element 31 of the oxygen sensor 19, that is, the element impedance detection circuit B (the part surrounded by the one-dot chain line in the figure) will be described later. The element impedance detection circuit B includes a CPU 43 and a first port 44, a first AD converter 45, a second port 46, and a second AD converter 47 that are input / output units thereof.

  In the ECU 22, the positive terminal 42 </ b> A is connected to a third AD converter 48 that is an input unit of the CPU 43, and the negative terminal 42 </ b> B is grounded. A filter circuit including a resistor 50 and a capacitor 51 is interposed between the positive terminal 42 </ b> A and the third AD converter 48. This filter circuit has a sufficiently large time constant and passes only the low frequency component of the voltage at the positive terminal 42A. For this reason, the third AD converter 48 can accurately generate a digital signal corresponding to the voltage value of the positive terminal 42 </ b> A while attenuating extreme noise and output the digital signal to the CPU 43. Since the negative electrode 33B of the oxygen sensor 19 is grounded via the negative electrode terminal 42B, the voltage value input to the CPU 43 through the third AD converter 48 becomes the value indicating the output voltage Vs of the oxygen sensor 19 as it is. Thus, the circuit corresponds to a sensor output voltage detection circuit. Hereinafter, the voltage input to the CPU 43 through the third AD converter 48 is referred to as a detection voltage VOX.

  In the ECU 22, an output detection resistor 57 is provided in parallel with the oxygen sensor 19. One end of the output detection resistor 57 is connected to the wiring connecting the filter circuit in the previous stage of the third AD converter 48 and the positive terminal 42A, and the other end is grounded. The output detection resistor 57 has a sufficiently large impedance compared to the element impedance Rs of the oxygen sensor 19.

  Next, the element impedance detection circuit B will be described. The element impedance detection circuit B includes a first switch element 71. The first switch element 71 is supplied with an element impedance detection voltage VOM having a predetermined magnitude. In the present embodiment, the element impedance detection voltage VOM has a magnitude of 5 V, which is also the power supply voltage of the ECU 22. The gate of the first switch element 71 is connected to the CPU 43 via the first port 44. The CPU 43 turns on the first switch element 71 by issuing an ON command as necessary.

  The first switch element 71 is connected to the first sampling point 73 via the first resistor 72. The first sampling point 73 is connected to the positive terminal 42 </ b> A via the second resistor 74 and is grounded via the capacitor 75.

  The first sampling point 73 is connected to the first AD converter 45 via a first intermediate resistor 83 and a filter circuit composed of a resistor 76 and a capacitor 77. This filter circuit has a sufficiently large time constant and passes only the low frequency component of the voltage at the first sampling point 73. Therefore, the first AD converter 45 can accurately generate a digital signal corresponding to the voltage value at the first sampling point 73 while attenuating extreme noise, and output the digital signal to the CPU 43.

  A second switch element 79 is connected to the first sampling point 73 via a third resistor 78. The second switch element 79 forcibly discharges the surplus charge supplied to the oxygen sensor 19 when the first switch element 71 is turned on, and returns the output voltage of the sensor after voltage application to the state before application. Is provided. The CPU 43 is connected to the gate of the second switch element 79 via the second port 46. The second switch element 79 is grounded. If necessary, the CPU 43 issues an ON command to turn on the second switch element 79 and ground the first sampling point 73 via the third resistor 78.

  A second sampling point 80 is formed between the second resistor 74 and the positive terminal 42A. The second sampling point 80 is connected to the second AD converter 47 via a second intermediate resistor 84 and a filter circuit composed of a resistor 81 and a capacitor 82. This filter circuit has a sufficiently large time constant and passes only the low frequency component of the voltage at the second sampling point 80. For this reason, the second AD converter 47 can accurately generate a digital signal corresponding to the voltage value of the second sampling point 80 while attenuating extreme noise and output the digital signal to the CPU 43. The second sampling point 80 also forms a branch point to the third AD converter 48 and is also connected to a filter circuit in the previous stage of the third AD converter 48.

  The first switch element 71 and the second switch element 79 are both turned off except when the element impedance Rs of the oxygen sensor 19 is detected. As a result, a voltage equal to the output voltage of the oxygen sensor 19 is generated at the second sampling point 80, and this voltage is input to the CPU 43 via the third AD converter 48. The CPU 43 determines whether the air-fuel ratio A / F of the exhaust gas is rich or lean based on the input voltage value VOX.

  Further, when the element impedance Rs of the oxygen sensor 19 is detected, the following operation is performed. First, the first switch element 71 is turned on with the second switch element 79 turned off. Then, the element impedance detection voltage VOM is applied, and the current I flows through the first resistor 72, the second resistor 74, and the oxygen sensor 19 that are in series with each other. At this time, the voltages at the first and second sampling points 73 and 80 at both ends of the second resistor 74 are input to the CPU 43 through the first AD converter 45 and the second AD converter 47, respectively. The CPU 43 obtains the current I from these voltage differences, that is, the voltage drop of the second resistor 74 and the known resistance value R2 of the second resistor 74. Next, the CPU 43 calculates the voltage drop of the first resistor 72 from the current I and the known resistance value R1 of the first resistor 72, and the voltage drop of the first resistor 72 and the second resistor 74 from the element impedance detection voltage VOM. Is subtracted to calculate a voltage drop in the element impedance Rs of the oxygen sensor 19. Finally, the CPU 43 calculates the element impedance Rs from the current I and the voltage drop in the element impedance Rs.

  Thus, when the calculation of the element impedance Rs is completed during the application of the element impedance detection voltage VOM, the CPU 43 turns off the first switch element 71 and turns on the second switch element 79 at the same time. Then, the application of the element impedance detection voltage VOM to the oxygen sensor 19 is cut off, and at the same time, the applied voltage is pulled back from the oxygen sensor 19. This is shown in FIG.

  The figure shows a change in the detection voltage VOX input to the CPU 43 through the third AD converter 48, which corresponds to a change in the potential of the positive electrode 33A of the oxygen sensor 19. As can be seen, the potential of the positive electrode 33A rises at the same time (ts) when the first switch element 71 is turned on, and the positive electrode 33A at the same time (th) when the first switch element 71 is turned off and the second switch element 79 is turned on. The potential of suddenly falls. Since the oxygen sensor 19 also includes a capacitive component, surplus charges remain in the oxygen sensor 19 and the potential of the positive electrode 33 </ b> A does not drop easily even if the first switch element 71 is turned on and then simply turned off ((B (See the figure). Therefore, the second switch element 79 is turned on to forcibly discharge this surplus charge. By doing so, the potential of the positive electrode 33A can be instantaneously dropped as illustrated.

  Here, the detection of the element impedance Rs does not affect normal control such as air-fuel ratio control, so that the periodic interval (T (n) −T (n−1) of normal control processing timing T (n), for example, 4 msec) within a very short time. Therefore, it is important to turn on the second switch element 79 and instantaneously drop the potential of the positive electrode 33A. Such an operation of forcibly discharging the surplus charge of the oxygen sensor 19 and instantaneously lowering the potential of the positive electrode 33A is referred to as “retraction”. The operation of applying the element impedance detection voltage VOM before pulling back is sometimes referred to as “sweep”. Since there is a voltage drop at the first resistor 72 and the second resistor 74, the peak voltage at the end of application th is slightly lower than the element impedance detection voltage VOM.

  The waveform shown in FIG. 8A is a normal waveform when neither the oxygen sensor 19 nor the element impedance detection circuit B has failed. On the other hand, when either one fails, the waveform changes as shown in FIG.

  FIG. 8B shows the case where the oxygen sensor 19 is normal and the element impedance detection circuit B is faulty. More specifically, the oxygen sensor 19 is normal and the second switch element 79 of the element impedance detection circuit B is faulty. Indicates. Hereinafter, this case is also simply referred to as “circuit failure”.

  As shown in the figure, when the second switch element 79 fails, the second switch element 79 is not turned ON at the time of pulling back, and the discharge of surplus charge is left to spontaneous discharge, so that the potential of the positive electrode 33A does not drop easily. Therefore, the potential of the positive electrode 33A, that is, the detection voltages VOX1 and VOX2, is obtained at the timings td1 and td2 before and after the application of the element impedance detection voltage VOM, and the difference or the absolute value of the difference is compared with a predetermined value to obtain the second value. A failure of the switch element 79 can be estimated once.

  That is, as shown in FIG. 8 (A), the detection voltages VOX1 and VOX2 are substantially equal in the normal state, whereas as shown in FIG. 8 (B), after the application of the detection voltage VOX1 before the application in the event of a circuit failure. Detection voltage VOX2 increases. Therefore, the difference between the detected voltages before and after application, that is, the voltage difference before and after application ΔVOX = VOX2−VOX1 or its absolute value is calculated. If the voltage difference before and after application ΔVOX or its absolute value is almost zero, the circuit is normal. It can be determined as a failure. Note that the timing td2 for obtaining the detection voltage VOX2 after application is after the end of pulling when normal as shown in FIG. 8A and at the time of pulling back when there is a failure as shown in FIG. 8B. Set to

  However, such a determination method cannot distinguish between a circuit failure and a sensor failure. FIG. 8C shows a case where the oxygen sensor 19 is faulty and the element impedance detection circuit B is normal. More specifically, the oxygen sensor 19 is faulty and the second switch element 79 of the element impedance detection circuit B is normal. Show the case. Hereinafter, this case is also simply referred to as “sensor failure”.

  As shown in the figure, when the sensor fails, the potential of the positive electrode 33A, that is, the detection voltage VOX changes due to the gas entering and exiting the detection element 31. Therefore, the detection voltages VOX1 and VOX2 before and after the application are not substantially equal values, and the detection voltage VOX2 after the application is higher or lower than the detection voltage VOX1 before the application. Therefore, the sensor failure can be estimated only by comparing the voltage difference before and after application ΔVOX or its absolute value with a predetermined value, and cannot be identified and distinguished from the circuit failure.

  As described above, in both cases of sensor failure and circuit failure, the same difference is found in the detected voltages VOX1 and VOX2 before and after the application. Therefore, any failure has occurred only by detecting these differences. It is not possible to specify.

  Therefore, in the present embodiment, paying attention to the difference in the characteristics of the detection voltage VOX according to the element impedance Rs of the oxygen sensor 19 and the intake air amount GA, the cases are divided into these regions, and the following is performed. Thus, the sensor failure and the circuit failure are distinguished and determined.

  First, the element impedance Rs of the oxygen sensor 19 will be described. In the oxygen sensor 19, the value of the element impedance Rs is a value correlated with the temperature of the detection element 31. As the temperature of the detection element 31 increases, the value of the element impedance Rs decreases. On the other hand, the oxygen sensor 19 cannot generate an output voltage unless the temperature of the detection element 31 reaches a high temperature corresponding to the activation temperature. Therefore, using this characteristic, a sensor failure and a circuit failure are distinguished and determined.

  That is, the ECU 22 determines whether or not the applied voltage difference ΔVOX or its absolute value is greater than a predetermined value when detecting the element impedance Rs. When it is determined that the value is larger than the predetermined value, the detected value of the element impedance Rs is compared with the predetermined value to determine as follows.

  First, when the value of the element impedance Rs is higher than a predetermined value, that is, when the detection element 31 is at a low temperature that is not at the activation temperature, the output voltage of the oxygen sensor 19 is not generated. The cause that the absolute value is larger than the predetermined value is considered to be the failure of the second switch element 79, and it is determined that the circuit is faulty. Accordingly, it is possible to prevent a true failure from being erroneously determined as a sensor failure while being a circuit failure, and it is possible to accurately determine a true circuit failure.

  Next, when the value of the element impedance Rs is lower than a predetermined value, that is, when the detection element 31 is at a high temperature that is at the activation temperature, it is determined that the sensor has failed. This is because when the detection element 31 is at a high temperature corresponding to the activation temperature, the element impedance Rs decreases, and even if the second switch element 79 fails and does not turn on, it is pulled back as shown in FIG. This is because, as shown in FIG. 8A, the pull-back is performed as quickly as in the normal state, and the voltage difference ΔVOX before and after the application or its absolute value becomes almost zero. Therefore, in such a case, the cause that the voltage difference ΔVOX before and after the application or the absolute value thereof becomes larger than the predetermined value is regarded as a failure of the oxygen sensor 19, and it is determined that the sensor is broken. Accordingly, it is possible to prevent a true failure from being erroneously determined as a circuit failure while being a sensor failure, and it is possible to accurately determine a true sensor failure.

  FIG. 9 shows the experimental results of examining the relationship between the element impedance Rs and the voltage difference ΔVOX before and after application. As will be understood from this, in the region where the element impedance Rs is smaller than 10000Ω (= 10 kΩ), the voltage difference ΔVOX before and after application becomes near zero in both cases of the circuit failure and the sensor failure, and the two cannot be distinguished. However, in the region where the element impedance Rs is greater than 10000Ω, the voltage difference ΔVOX before and after application is still near zero in the case of a sensor failure, whereas the voltage before and after application is emphasized by a broken line ellipse in the case of a circuit failure. The difference ΔVOX becomes a value of about 0.5 V or more which is greater than zero, and shows a tendency to increase as the element impedance Rs increases. Therefore, in the region where the element impedance Rs is greater than 10000Ω, that is, in the low temperature region of the oxygen sensor, it is possible to distinguish and specify a circuit failure from a sensor failure.

  FIG. 10 also shows the experimental results of examining the relationship between the element impedance Rs and the voltage difference ΔVOX before and after the application. The difference from FIG. 9 is in the range of the element impedance Rs, and FIG. 10 shows a low range of 300Ω or less, that is, a value only when the oxygen sensor is at a high temperature. As will be understood from this, the pre-applied voltage difference ΔVOX is always near zero when the oxygen sensor is normal, but the applied pre-applied voltage is emphasized by a broken-line circle when the oxygen sensor is out of order. The difference ΔVOX can be a value greatly deviating from zero. Therefore, it is possible to specify a sensor failure at a high temperature of the oxygen sensor.

  Next, the intake air amount GA will be described. FIG. 11 shows experimental results obtained by examining changes with time in (A) vehicle speed, (B) intake air amount GA, and (C) detection voltage VOX. The intake air amount GA is a value detected by the air flow meter 16. It is assumed that there is no circuit failure with respect to the detection voltage VOX. Further, it is assumed that the element impedance Rs is a low value such that the detection element 31 reaches the activation temperature. In the figure, (i) is a value with a low intake air amount GA, (ii) is a value with a medium intake air amount GA, and (iii) is a value with a high intake air amount GA. This is the case. Regarding the detection voltage VOX, the lines P and Q indicate the normal time and the failure time of the oxygen sensor 19, respectively.

  As shown in the figure, the waveform of the detection voltage VOX is different between when the sensor is normal and when it is malfunctioning, and the difference is only seen when (ii) the intake air amount GA is a medium value. In the case of a sensor failure, when the intake air amount GA is a medium value, gas enters and exits the detection element as described above, and the value of the sensor output voltage, that is, the detection voltage VOX varies. In other words, sensor noise comes on the waveform of the detection voltage VOX.

  However, even in the case of a sensor failure, (i) such fluctuations are not seen when the intake air amount GA is a low value. The reason is that when the amount of air is low such as idling, the exhaust gas does not enter the detection element from the defective portion of the detection element, and therefore gas exchange is not performed. Further, (iii) such fluctuations are not observed when the intake air amount GA is a high value. The reason for this is that when the amount of air is high, such as acceleration, exhaust gas can enter the detection element from the defect portion of the detection element, but cannot exit from the detection element, and therefore gas exchange is not performed. It is. As described above, the sensor output fluctuates only when the amount of air is such that the gas exchange to the detection element is allowed, and on the other hand, the low air quantity or the high air quantity that does not allow the gas exchange to the detection element. In this case, the sensor output does not fluctuate.

  Here, the magnitude of the fluctuation of the sensor output can be obtained by calculating the fluctuation integrated amount Mvox of the detection voltage VOX (= oxygen sensor output voltage Vs) by the method described with reference to FIG.

  Therefore, in this embodiment, failure diagnosis is performed as follows using such characteristics. That is, when the detected element impedance Rs is smaller than a predetermined value (that is, when the oxygen sensor is at a high temperature), the ECU 22 detects that the detected intake air amount GA is a medium value, and When the value of the calculated fluctuation integration amount Mvox is larger than a predetermined value (that is, when the sensor output fluctuation is large), it is determined that the sensor is faulty. On the other hand, when the value of the detected intake air amount GA is a low or high value and the voltage difference ΔVOX before and after application or the absolute value thereof is larger than a predetermined value, it is determined that a circuit failure has occurred.

  Next, a further specific example of the failure diagnosis process as described above will be described with reference to FIG. FIG. 12 is a flowchart of a routine for executing the failure diagnosis process. This routine is repeatedly executed by the ECU 22 every predetermined cycle (for example, 4 msec), and more specifically, the processing timing as shown in FIG. 8 or FIG. It is executed every T (n).

  First, in step S101, the detection voltage VOX is acquired. Next, in step S102, the detection voltage VOX1 before application of the element impedance detection voltage VOM and the detection voltage VOX2 after application of the element impedance detection voltage VOM detected at the timing closest to the processing timing T (n) are acquired. (See FIG. 8). Further, in step S103, a voltage difference ΔVOX before and after application (= VOX2−VOX1) is calculated using the detected voltages VOX1 and VOX2 before and after the application. Thereafter, in step S104, the element impedance Rs calculated when the element impedance detection voltage VOM is applied is acquired.

  Next, in step S105, it is determined whether or not the element impedance Rs is larger than a predetermined first threshold value Rs1, that is, whether or not the detection element temperature of the oxygen sensor is low. The first threshold value Rs1 is set to 10000Ω based on the experimental result of FIG.

  When the element impedance Rs is larger than the first threshold value Rs1, the process proceeds to step S106, and it is determined whether or not the voltage difference ΔVOX before and after application is larger than a predetermined first threshold value ΔVOXs1. The first threshold ΔVOXs1 is set to 0.5 V, for example, based on the experimental result of FIG.

  When the voltage difference ΔVOX before and after application is larger than the first threshold value ΔVOXs1, the process proceeds to step S107 and it is determined that a circuit failure has occurred.

  In step S108, the value of the detection voltage VOX acquired in step S101 is saved, and this routine is terminated.

  On the other hand, if it is determined in step S105 that the element impedance Rs is equal to or lower than the first threshold value Rs1, that is, if it is determined that the detection element temperature of the oxygen sensor is not low, in step S109, the element impedance Rs is a predetermined first value. It is determined whether it is smaller than 2 threshold value Rs2. The second threshold value Rs2 is set to a value smaller than the first threshold value Rs1 in step S105. More specifically, when the element impedance Rs is smaller than the second threshold value Rs2, the detection element temperature of the oxygen sensor is The value is such that the activation temperature is high. The second threshold value Rs2 is set to 500Ω based on the experimental result of FIG.

  When the element impedance Rs is smaller than the second threshold value Rs2, the intake air amount GA detected by the air flow meter 16 is smaller than the predetermined first threshold value GAs1 or the predetermined second threshold value GAs2 in step S110. It is determined whether it is larger. Here, GAs1 <GAs2, for example, the first threshold GAs1 is 5 g / sec, and the second threshold GAs2 is 15 g / sec. In step S110, it is substantially determined whether the intake air amount GA is a low air amount or a high air amount.

  When the intake air amount GA is smaller than the first threshold value GAs1 or larger than the second threshold value GAs2, that is, when the intake air amount GA is a low air amount or a high air amount, in step S111, a voltage difference before and after application. It is determined whether or not the absolute value of ΔVOX is greater than a predetermined second threshold value ΔVOXs2. The second threshold value ΔVOXs2 is set to a value smaller than the first threshold value ΔVOXs1 in step S106, for example, 0.1V.

  When the absolute value of the voltage difference ΔVOX before and after application is larger than the second threshold value ΔVOXs2, the process proceeds to step S112 and it is determined that a circuit failure has occurred. That is, when the amount of air is low or high, even if the oxygen sensor is out of order, there is no exchange of gas to the detection element and no sensor noise occurs. Therefore, in this case, the cause that the absolute value of the voltage difference ΔVOX before and after the application becomes larger than the second threshold value ΔVOXs2 can be regarded as a failure of the second switch element 79. Therefore, it is determined here as a circuit failure. Thereafter, in step S108, the value of the detected voltage VOX is stored, and this routine is terminated.

  On the other hand, if it is determined in step S109 that the element impedance Rs is greater than or equal to the second threshold value Rs2, this routine is terminated. That is, when the element impedance Rs is equal to or higher than the second threshold value Rs2 and equal to or lower than the first threshold value Rs1, failure diagnosis is not performed because the sensor element temperature is at an intermediate temperature before the activation temperature.

  In step S110, if the intake air amount GA is not less than the first threshold GAs1 and not more than the second threshold GAs2, that is, if the amount of air is medium, the process proceeds to step S113.

  In step S113, a counter built in CPU 43 is counted up. In step S114, the fluctuation integration amount Mvox is calculated based on the detected voltage VOX. In the next step S115, it is determined whether or not the counter has exceeded a predetermined time tks. The predetermined time tks corresponds to an integration time for calculating the fluctuation integration amount Mvox, and is set to 10 sec, for example. If the counter does not exceed the predetermined time tks (that is, if the integration has not ended), the routine ends through step S108, and if the counter exceeds the predetermined time tks (that is, if the integration has ended), step S116. Proceed to

  In step S116, it is determined whether or not the finally calculated fluctuation integration amount Mvox exceeds a predetermined threshold value Mvoxs, that is, whether or not the fluctuation amount of the sensor output voltage is large enough to correspond to the sensor failure time. Is done. The threshold value Mvoxs is, for example, 10V. When it is determined that the fluctuation accumulated amount Mvox does not exceed the predetermined threshold value Mvoxs, the routine is ended through step S108.

  On the other hand, when it is determined that the fluctuation integration amount Mvox exceeds the predetermined threshold value Mvoxs, it is determined in step S117 that the sensor is faulty. In step S118, the counter is reset. In step S108, the current detection voltage VOX is stored, and the routine is terminated.

  As described above, according to the present embodiment, the sensor failure and the circuit failure can be distinguished and determined by dividing the case according to the element impedance of the oxygen sensor and / or the magnitude of the intake air amount. As a result, it is possible to specify a faulty part and improve the accuracy of fault diagnosis.

  In the present embodiment, the ECU 22 constitutes impedance reference failure determination means, air amount reference failure determination means, and impedance / air amount reference failure determination means.

  The present invention can take other embodiments. For example, the numerical values used in the embodiments can be arbitrarily changed. The internal combustion engine is not limited to being mounted on a vehicle, and the arrangement method and installation position of the oxygen sensor can be arbitrarily changed.

  The present invention is effective for a case where a circuit failure occurs such that a voltage can be applied but cannot be pulled back when detecting the element impedance. Accordingly, the failure of the element impedance detection circuit can include, for example, a disconnection between the pull-back switch element (second switch element 79) and the ground point.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

It is a figure which shows the structure of the exhaust gas purification system of the vehicle-mounted internal combustion engine which concerns on this embodiment. It is sectional drawing which shows the attachment state of an oxygen sensor. It is an expanded sectional view of the detection element periphery of an oxygen sensor. It is a graph which shows the output characteristic of an oxygen sensor. It is an expanded sectional view when a defective part arises in a detection element of an oxygen sensor. It is a figure for demonstrating the calculation method of a fluctuation | variation integration amount. It is a figure which shows the structure of the electric circuit containing an oxygen sensor and ECU. It is a figure which shows the change of the detection voltage at the time of element impedance detection. It is a graph which shows the experimental result which investigated the relationship between element impedance and the voltage difference before and behind application. It is a graph which shows the experimental result which investigated the relationship between element impedance and the voltage difference before and behind application. It is a graph which shows the experimental result which investigated the time-dependent change of (A) vehicle speed, (B) intake air amount, and (C) detection voltage. It is a flowchart of a routine for executing failure diagnosis processing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 13 Exhaust passage 19, 20 Oxygen sensor 22 Electronic control unit (ECU)
31 Detection element 43 Central processing unit (CPU)
79 Second switch element B Element impedance detection circuit Rs Element impedance Rs1 Element impedance first threshold value Rs2 Element impedance second threshold value VOM Element impedance detection voltage VOX Detection voltage VOX1 Detection voltage VOX2 before application Detection voltage after application ΔVOX Pre- and post-application voltage difference ΔVOXs1 Pre-application voltage difference first threshold ΔVOXs2 Pre-application voltage difference second threshold GA Intake air amount GA1 Intake air amount first threshold GA2 Intake air amount second threshold Value Mvox Fluctuation integration amount Mvoxs Fluctuation integration amount threshold

Claims (8)

In an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas,
Applying a predetermined element impedance detection voltage to the oxygen sensor and then pulling it back, and an element impedance detection circuit for detecting the element impedance of the oxygen sensor in this application process;
A sensor output voltage detection circuit for detecting an output voltage of the oxygen sensor;
When the voltage difference of the oxygen sensor output voltage before and after the application detected by the sensor output voltage detection circuit or the absolute value thereof is larger than a predetermined value, at least according to the value of the element impedance detected by the element impedance detection circuit, An oxygen sensor failure diagnosis device, comprising: an impedance reference failure determination unit that distinguishes and determines a failure of the oxygen sensor and a failure of the element impedance detection circuit. The oxygen sensor failure diagnosis apparatus according to claim 1, wherein the impedance reference failure determination means determines that the device impedance detection circuit has failed when a value of the detected device impedance is greater than a predetermined value. The oxygen sensor failure diagnosis device according to claim 1, wherein the impedance reference failure determination unit determines that the oxygen sensor is in failure when a value of the detected element impedance is smaller than a predetermined value. In an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas,
Applying a predetermined element impedance detection voltage to the oxygen sensor and then pulling it back, and an element impedance detection circuit for detecting the element impedance of the oxygen sensor in this application process;
A sensor output voltage detection circuit for detecting an output voltage of the oxygen sensor;
Intake air amount detection means for detecting the intake air amount of the internal combustion engine;
When the value of the element impedance detected by the element impedance detection circuit is smaller than a predetermined value, a failure of the oxygen sensor according to at least the value of the intake air amount detected by the intake air amount detection means, and the element An oxygen sensor failure diagnosis device comprising: an air amount reference failure determination unit that distinguishes and determines a failure of an impedance detection circuit. A variable integrated amount calculating means for calculating a variable integrated amount of the output voltage of the oxygen sensor;
The air amount reference failure determining means has a medium value of the detected intake air amount, and when the value of the fluctuation integrated amount calculated by the fluctuation integrated amount calculating means is larger than a predetermined value, The oxygen sensor failure diagnosis device according to claim 4, wherein the oxygen sensor failure is determined. The air amount reference failure determination means has a value of the detected intake air amount being a low or high value, and the voltage of the oxygen sensor output voltage before and after the application detected by the sensor output voltage detection circuit. 6. The oxygen sensor failure diagnosis device according to claim 4, wherein when the difference or the absolute value thereof is larger than a predetermined value, it is determined that the element impedance detection circuit has failed. In an oxygen sensor failure diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas,
Applying a predetermined element impedance detection voltage to the oxygen sensor and then pulling it back, and an element impedance detection circuit for detecting the element impedance of the oxygen sensor in this application process;
A sensor output voltage detection circuit for detecting an output voltage of the oxygen sensor;
Intake air amount detection means for detecting the intake air amount of the internal combustion engine;
Fluctuation integration amount calculation means for calculating a fluctuation integration amount of the detected output voltage of the oxygen sensor;
At least one of the value of the element impedance detected by the element impedance detection circuit and the value of the intake air amount detected by the intake air amount detection means, and the oxygen sensor before and after the application detected by the sensor output voltage detection circuit Based on the voltage difference of the output voltage or the absolute value thereof and at least one of the values of the fluctuation integration amount calculated by the fluctuation integration amount calculation means, the failure of the oxygen sensor is distinguished from the failure of the element impedance detection circuit. An oxygen sensor failure diagnosis device comprising: an impedance / air amount reference failure determination means for determining. The oxygen sensor failure diagnosis apparatus according to any one of claims 1 to 7, wherein the failure of the element impedance detection circuit is a failure of a switch element that is turned on when the voltage is pulled back from the oxygen sensor.

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