JP3633055B2 - Engine diagnostic equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an engine diagnostic device, and more particularly to a catalyst or air-fuel ratio diagnostic device for a lean engine.
[0002]
[Prior art]
Conventionally, in an apparatus disclosed in, for example, Japanese Patent Application Laid-Open No. 4-116239, a three-way catalyst is interposed in an engine exhaust passage, and the O ₂ concentration in the exhaust gas is increased on the upstream side and the downstream side of the three-way catalyst. fuel from the upstream O ₂ sensor and the downstream O ₂ sensor is provided, the upstream O ₂ fuel injection valve so that a predetermined air-fuel ratio is obtained based on the output signal of the sensor that outputs the rich lean inversion signal in response while performing the air-fuel ratio feedback control to increase or decrease correcting the injection amount, using the upstream O ₂ sensor and the output value of the downstream O ₂ sensor when the air-fuel ratio feedback control, i.e., the output signal of the upstream O ₂ sensor a rich lean inversion frequency f1, on the basis of the downstream O ₂ rich lean reversal frequency f2 of the output signal of the sensor, determine the inversion frequency ratio f2 / f1, der the inversion frequency ratio is less than the set value Sometimes the three-way catalyst is normal, it is larger than the set value is to be diagnosed as abnormal (deteriorated).
[0003]
[Problems to be solved by the invention]
By the way, in an engine (lean engine) provided with air-fuel ratio switching means for switching the air-fuel ratio of the air-fuel mixture supplied to the engine between a stoichiometric air-fuel ratio (stoichiometric) and a lean air-fuel ratio (lean) according to operating conditions, NOx is adsorbed during operation at a lean air-fuel ratio (during lean operation), and a NOx adsorption catalyst capable of reducing the adsorbed NOx during operation at stoichiometric air-fuel ratio (during stoichiometric operation) is provided, or during stoichiometric operation A lean NOx catalyst that can adsorb HC and reduce NOx by the adsorbed HC (and HC newly discharged from the engine) during lean operation is provided, and a NOx adsorption catalyst is provided downstream thereof.
[0004]
However, such a catalyst for a lean engine cannot be diagnosed by the conventional diagnostic device.
That is, the NOx adsorption performance of the NOx adsorption catalyst cannot be determined for deterioration by the O ₂ concentration, and the NOx conversion performance of the lean NOx catalyst cannot be determined for deterioration by the O ₂ concentration.
[0005]
The present invention has been made in view of such conventional problems, and an object thereof is to enable diagnosis of a catalyst or the like for a lean engine.
[0006]
[Means for Solving the Problems]
Therefore, in the invention according to claim 1, as shown in FIG. 1 (A), in the engine provided with the NOx adsorption catalyst 8 in the exhaust passage 5, NOx for detecting NOx in the exhaust downstream of the NOx adsorption catalyst 8 is detected. a sensor 9, while providing a temperature sensor 10 for detecting the temperature of the NOx trap catalyst 8, stoichiometric → lean after switching, the time until the output value of the NOx sensor 9 exceeds a predetermined slice level, and the catalyst temperature Is determined as deterioration of the NOx adsorption catalyst 8 when the time until the slice level is exceeded is less than a predetermined value and the accumulated time is greater than a predetermined value, When the time until the slice level is exceeded is smaller than a predetermined value and the accumulated time is smaller than the predetermined value, it is determined that the NOx adsorption catalyst 8 is temporarily deteriorated due to S poisoning. A NOx adsorption catalyst deterioration determining means is provided to constitute a catalyst diagnostic apparatus.
[0007]
In other words, as the adsorption performance of the NOx adsorption catalyst deteriorates, the time for the output value of the NOx sensor to reach the slice level after the stoichiometric to lean change becomes shorter. By detecting this, the deterioration of the adsorption performance of the NOx adsorption catalyst is diagnosed. To do.
Further, if a large amount of S is deposited on the surface of the NOx adsorption catalyst, NOx is not adsorbed and the amount of NOx discharged increases. However, the deposition of S is temporary, and if the catalyst is held at a high temperature, S decomposes. Thus, it is known that the amount of S deposited is reduced. Therefore, in consideration of the catalyst temperature, diagnosis is performed by classifying permanent deterioration due to heat and temporary deterioration due to S poisoning of the NOx adsorption catalyst.
[0008]
In the invention according to claim 2 , as shown in FIG. 1 (B), in the engine provided with the lean NOx catalyst 7 in the exhaust passage and further provided with the NOx adsorption catalyst 8 on the downstream side thereof, downstream of the NOx adsorption catalyst 8. exceeds the NOx sensor 9 for detecting the NOx in the exhaust gas, while providing a temperature sensor 10 for detecting the temperature of the NOx trap catalyst 8, the換後stoichiometric → lean switching, the slice level output values are set in advance of the NOx sensor 9 NOx adsorption when the time until the slice level is exceeded is less than the prescribed value and the accumulated time is greater than the prescribed value based on the accumulated time during which the catalyst temperature is in a high temperature state equal to or higher than the prescribed value. When it is determined that the catalyst 8 has deteriorated and the time until the slice level is exceeded is smaller than a predetermined value and the accumulated time is smaller than the predetermined value, the NOx adsorption catalyst NOx adsorption catalyst deterioration determining means for determining temporary deterioration due to S poisoning of the medium 8 and lean NOx catalyst deterioration determining means for determining deterioration of the lean NOx catalyst 7 based on the level of the output value of the NOx sensor 9 are provided. Thus, a catalyst diagnostic device is constructed.
[0009]
That is, as the adsorption performance of the NOx adsorption catalyst deteriorates, the time τ for the output value of the NOx sensor to reach the slice level after the stoichiometric to lean change becomes shorter, and as the NOx conversion performance of the lean NOx catalyst deteriorates, Since the level of the output value increases, the deterioration of the adsorption performance of the NOx adsorption catalyst and the deterioration of the conversion performance of the lean NOx catalyst are diagnosed by detecting them.
According to this, the permanent deterioration due to heat of the NOx adsorption catalyst, the temporary deterioration due to S poisoning, and the deterioration of the lean NOx catalyst can be classified and diagnosed.
[0010]
In the invention according to claim 3 , in the engine provided with the catalyst in the exhaust passage, the upstream NOx sensor 11 for detecting NOx in the exhaust is provided upstream of the catalyst, while the upstream NOx sensor is in operation at the lean air-fuel ratio. When the output value is larger than the upper limit value set according to the air-fuel ratio, it is determined that the air-fuel ratio is richly shifted. When the output value is smaller than the lower limit value set according to the air-fuel ratio, the air-fuel ratio is lean-shifted. An air-fuel ratio shift determining means for determining that the air-fuel ratio is present is provided to constitute an air-fuel ratio diagnosis apparatus (see FIGS. 1A and 1B).
The amount of NOx emitted from the vehicle depends not only on the performance of the catalyst but also on the amount of NOx emitted from the engine. In general, the NOx emission amount is determined by the air-fuel ratio, but there is a range of variations. If the upper limit value of this range is exceeded, a rich shift of the air-fuel ratio can be diagnosed, and if it is less than the lower limit value, a lean shift of the air-fuel ratio can be diagnosed. Therefore, the air-fuel ratio shift is diagnosed based on the output level of the upstream NOx sensor.
In the invention according to claim 4 , in addition to the inventions according to claims 1 and 2 , the upstream NOx sensor 11 for detecting NOx in the exhaust gas is provided upstream of the catalyst, while the upstream NOx sensor 11 is provided during lean operation. The air-fuel ratio shift determining means for determining the air-fuel ratio shift based on the output value level is provided (see FIGS. 1A and 1B).
According to this, in addition to the actions of the inventions according to claims 1 and 2, it is possible to diagnose the shift of the air-fuel ratio.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
[First embodiment]
FIG. 2 is a system diagram.
A fuel injection valve 3 is provided in the intake passage 2 of the engine 1, and this fuel injection valve 3 is controlled by a control unit 4. The exhaust passage 5 is provided with an O ₂ sensor 6 on the upstream side of a catalyst to be described later, and its signal is input to the control unit 4.
[0013]
The control unit 4 switches the target air-fuel ratio between stoichiometric and lean according to the engine operating conditions. Under the stoichiometric conditions, the fuel injection valve 3 is controlled by feedback control based on a signal from the O ₂ sensor 6 and under lean conditions by open control. The amount of fuel injection is controlled.
For exhaust purification, a NOx adsorption catalyst 8 is interposed in the exhaust passage 5. The NOx adsorption catalyst 8 can adsorb NOx during the lean operation and can reduce the adsorbed NOx during the stoichiometric operation.
[0014]
Here, a NOx sensor 9 for detecting NOx in the exhaust gas is provided on the downstream side of the NOx adsorption catalyst 8, and the signal is input to the control unit 4.
The NOx sensor 9 uses an oxide semiconductor such as In ³⁺ —TiO ₂ , WO ₃ , or Sc ₂ O ₃ —CuO.
FIG. 3 shows the NOx sensor output at the time of switching from stoichiometric to lean. The NOx adsorption performance of the NOx adsorption catalyst results in the characteristics shown in the figure, and the time τ to reach the slice level becomes shorter as the NOx adsorption performance deteriorates. Therefore, the deterioration of the adsorption performance of the NOx adsorption catalyst can be diagnosed by detecting this.
[0015]
Also, to order to diagnose and classify a temporary deterioration in the S poisoning of the NOx trap catalyst, to provide a temperature sensor 10 for detecting the temperature of the NOx trap catalyst 8, and input the signals to the control unit 4 is there.
FIG. 4 shows NOx emission characteristics due to S poisoning of the NOx adsorption catalyst. When a large amount of S accumulates on the catalyst surface, BaNO ₃ and NOx are originally adsorbed when lean , and BaSO ₄ is absorbed, and NOx is not adsorbed, resulting in an increase in NOx emission. However, it is known that the deposition of S is temporary, and if the catalyst is held for a certain period of time (for example, at 600 ° C. for 30 minutes), S is decomposed and the amount of deposited S is reduced. Therefore, by detecting whether the catalyst has been in a high temperature environment for a certain period of time, it is possible to classify and diagnose the permanent deterioration due to heat and the temporary deterioration due to S poisoning of the NOx adsorption catalyst.
[0016]
FIG. 5 shows a diagnosis flowchart.
In step 101 (denoted as S 101 in the figure, the same applies hereinafter), the catalyst temperature T cat is read based on the signal from the temperature sensor 10 . In step 102 , the catalyst temperature T cat is compared with a predetermined value T _0, and if T cat ≧ T ₀ , the counter TM is incremented in step 103 . Therefore, the counter TM indicates the accumulated time in which the catalyst temperature T cat is in a high temperature state equal to or higher than the predetermined value T ₀ after the engine is started .
[0017]
In step 1 , it is determined whether or not the lean condition is satisfied, and the process proceeds to step 2 only when the lean condition is satisfied.
In step 2, it is determined whether or not the lean condition is also the previous time. If NO, the counter CNT that counts the elapsed time after the switching is cleared in step 3 at the time of switching from stoichiometric to lean. In the case of YES, the counter CNT is incremented in step 4.
[0018]
In step 5, the output value NOxV of the NOx sensor 9 is read.
In step 6, the output value NOxV of the NOx sensor 9 is compared with a predetermined value (slice level) _{V 0,} in the case of NOxV _{<V 0,} is determined to be normal NOx adsorption catalyst in step 8.
If NOxV ≧ V ₀ , the counter CNT is compared with the predetermined value C ₀ in step 7, and if CNT ≧ C ₀ , it is determined that the NOx adsorption catalyst is normal in step 8, but if CNT <C ₀ compares the counter TM with a predetermined value TM ₀ in step 104.
Result of the comparison, in the case of TM ≧ TM _0, it is determined that the NOx adsorption catalyst deteriorates in step 9, in the case of TM <TM _0, determines that the S poisoning at step 105.
Here, steps 1 to 9 and 101 to 105 correspond to NOx adsorption catalyst deterioration determination means.
[0019]
[Second Embodiment]
In this embodiment, a lean NOx catalyst + NOx adsorption catalyst is provided.
FIG. 6 is a system diagram.
In the exhaust passage 5, a lean NOx catalyst 7 and a NOx adsorption catalyst 8 are interposed in series. The lean NOx catalyst 7 can adsorb HC during the stoichiometric operation, and can reduce NOx by the adsorbed HC and the HC newly discharged from the engine during the lean operation. The NOx adsorption catalyst 8 can adsorb NOx during the lean operation and can reduce the adsorbed NOx during the stoichiometric operation.
[0020]
Here, a NOx sensor 9 for detecting NOx in the exhaust gas is provided on the downstream side of the NOx adsorption catalyst 8, and the signal is input to the control unit 4.
FIG. 7 shows the NOx sensor output at the time of switching from stoichiometric to lean. As the NOx adsorption performance of the NOx adsorption catalyst deteriorates, the time τ to reach the slice level is shortened, and the output level increases as the NOx conversion performance of the lean NOx catalyst deteriorates. Therefore, the deterioration of the adsorption performance of the NOx adsorption catalyst can be diagnosed by the time τ, and the conversion performance deterioration of the lean NOx catalyst can be diagnosed by the output level.
[0021]
In addition, a temperature sensor 10 that detects the temperature of the NOx adsorption catalyst 8 is provided and the signal is input to the control unit 4 so that temporary deterioration due to S poisoning of the NOx adsorption catalyst can be classified and diagnosed. . Although the temperature sensor 10 is provided between the lean NOx catalyst 7 and the NOx adsorption catalyst 8 in the drawing, it may be at any position where the temperature of the NOx adsorption catalyst 8 (or a temperature related thereto) can be detected.
[0022]
FIG. 8 shows a diagnosis flowchart.
In step 101 , the catalyst temperature T cat is read based on the signal from the temperature sensor 10 . In step 102 , the catalyst temperature T cat is compared with a predetermined value T _0, and if T cat ≧ T ₀ , the counter TM is incremented in step 103 . Therefore, the counter TM indicates the accumulated time in which the catalyst temperature T cat is in a high temperature state equal to or higher than the predetermined value T ₀ after the engine is started .
[0023]
In step 1, it is determined whether or not the lean condition is satisfied, and the process proceeds to step 2 only when the lean condition is satisfied.
In step 2, it is determined whether or not the lean condition is also the previous time. If NO, the counter CNT that counts the elapsed time after the switching is cleared in step 3 at the time of switching from stoichiometric to lean. In the case of YES, the counter CNT is incremented in step 4.
[0024]
In step 5, the output value NOxV of the NOx sensor 9 is read.
In step 11, the output value NOxV of the NOx sensor 9 is compared with a predetermined value (slice level) _{V 0,} in the case of NOxV _{<V 0,} is determined to be normal NOx adsorption catalyst and the lean NOx catalyst in step 14.
If NOxV ≧ V ₀ , the output value NOxV of the NOx sensor 9 is compared with a predetermined value V ₁ (> V ₀ ) in step 12, and if NOxV <V ₁ , the process proceeds to step 13.
In step 13, compares the counter CNT and the predetermined value _{C 0,} in case of CNT ≧ _{C 0,} is determined to be normal NOx adsorption catalyst and the lean NOx catalyst in step 14, in the case of CNT _{<C 0,} step 104 Proceed to
[0025]
For NOxV ≧ _{V 1,} determines that the lean NOx catalyst deterioration in a step 15. Then, the process proceeds to step 16.
In step 16, it compares the counter CNT and the predetermined value _{C 0,} in case of CNT _{<C 0,} the process proceeds to step 104.
[0026]
In step 104 compares the counter TM with a predetermined value TM _0.
Result of the comparison, in the case of TM ≧ TM _0, it is determined that the NOx adsorption catalyst deteriorates at step 17, in the case of TM <TM _0, determines that the S poisoning at step 105.
Here, steps 1 to 5, 11 , 13 , 14 , 16 , 17 , 101 to 105 correspond to NOx adsorption catalyst deterioration determination means, and steps 12 and 15 correspond to lean NOx catalyst deterioration determination means. .
[0027]
[ Third embodiment]
In this embodiment, an air-fuel ratio shift that affects the amount of NOx discharged from the engine is diagnosed.
FIG. 9 is a system diagram.
A NOx adsorption catalyst 8 (or lean NOx catalyst 7) is interposed in the exhaust passage 5.
Here, an upstream NOx sensor 11 for detecting NOx in the exhaust gas is provided upstream of the NOx adsorption catalyst 8 (substantially the same position as the O ₂ sensor 6), and the signal is input to the control unit 4.
[0028]
FIG. 10 shows the relationship between the air / fuel ratio (A / F) of the engine and NOx.
In general, the NOx emission amount decreases as the A / F becomes leaner, but there is a variation Δl in the NOx emission amount due to engine variations, particularly cylinder A / F variation. If a NOx value exceeding Δl is indicated, this means that A / F has changed. For example, if the upper limit value max is exceeded, a rich shift is performed, and if it is less than the lower limit value min, a lean shift is performed. Therefore, it is possible to diagnose the A / F shift, not the catalyst.
[0029]
FIG. 11 shows a diagnosis flowchart.
In step 200, it is determined whether or not the lean condition is satisfied. The process proceeds to step 201 only when the lean condition is satisfied.
In step 201, the output value V ′ of the upstream NOx sensor 11 is read.
In step 202, the output value V of the upstream side NOx sensor 11 'is compared with the upper limit value _{V max,} V' in the case of ≧ _{V max,} determines that the A / F rich shift in a step 203.
If V ′ <V _max , the output value V ′ of the upstream NOx sensor 11 is compared with the lower limit value V _min in step 204, and if V ′ ≦ V _min , it is determined as A / F lean shift in step 205. To do.
If V ′> V _min , it is determined in step 206 that the A / F is normal.
Here, steps 200 to 206 correspond to air-fuel ratio shift determining means.
[0030]
[ Fourth embodiment]
This embodiment is a combination of the first embodiment and the third embodiment.
FIG. 12 is a system diagram.
The difference from the first embodiment ( FIG. 2 ) is that an upstream NOx sensor 11 for detecting NOx in the exhaust gas is provided on the upstream side of the NOx adsorption catalyst 8 (substantially the same position as the O ₂ sensor 6). Is input to the control unit 4.
[0031]
FIG. 13 shows a diagnosis flowchart.
The difference from the first embodiment (FIG. 5) is that processing (air-fuel ratio shift determining means) in steps 201 to 206 is added in the middle.
[0032]
In step 201 , the output value V ′ of the upstream NOx sensor 11 is read.
In step 202, the output value V of the upstream side NOx sensor 11 'is compared with the upper limit value _{V max,} V' in the case of ≧ _{V max,} determines that the A / F rich shift in a step 203.
If V ′ <V _max , the output value V ′ of the upstream NOx sensor 11 is compared with the lower limit value V _min in step 204 , and if V ′ ≦ V _min , it is determined in step 205 that the A / F lean shift. To do.
If V ′> V _min , it is determined in step 206 that the A / F is normal.
Here, steps 201 to 206 correspond to air-fuel ratio shift determining means.
[0033]
[ Fifth embodiment]
This embodiment is a combination of the second embodiment and the third embodiment.
FIG. 14 is a system diagram.
The difference from the second embodiment ( FIG. 6 ) is that an upstream NOx sensor 11 for detecting NOx in the exhaust gas is provided upstream of the lean NOx catalyst 7 (substantially at the same position as the O ₂ sensor 6). Is input to the control unit 4.
[0034]
FIG. 15 shows a diagnosis flowchart.
The difference from the second embodiment ( FIG. 8 ) is that processing (air-fuel ratio shift determining means) in steps 201 to 206 is added in the middle.
The processing contents of steps 201 to 206 are the same as the processing contents in the fourth embodiment.
[0035]
【The invention's effect】
As described above, according to the first aspect of the present invention, the deterioration of the adsorption performance of the NOx adsorption catalyst is classified into the deterioration due to heat of the NOx adsorption catalyst and the temporary deterioration due to S poisoning, and is accurately diagnosed. The effect that it can be obtained.
According to the second aspect of the present invention, the deterioration of the adsorption performance of the NOx adsorption catalyst can be classified into the deterioration of the NOx adsorption catalyst due to heat and the temporary deterioration due to S poisoning, and can be accurately diagnosed. The effect that deterioration of conversion performance can be diagnosed is obtained.
[0036]
According to the third aspect of the present invention, an effect of accurately diagnosing an air-fuel ratio shift that greatly affects the amount of NOx discharged from the engine can be obtained.
According to the invention of claim 4 , in addition to the effects of the inventions of claims 1 to 2, there is an effect that an air-fuel ratio shift that greatly affects the amount of NOx discharged from the engine can be accurately diagnosed. can get.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration of the present invention. FIG. 2 is a system diagram showing a first embodiment of the invention. FIG. 3 is a characteristic diagram of the first embodiment.
FIG. 4 is a characteristic diagram of the first embodiment.
FIG. 5 is a flowchart of diagnosis of the first embodiment.
FIG. 6 is a system diagram showing a second embodiment of the present invention.
FIG. 7 is a characteristic diagram of the second embodiment.
FIG. 8 is a diagnosis flowchart of the second embodiment;
FIG. 9 is a system diagram showing a third embodiment of the present invention.
[10] Ibid characteristic diagram of the third embodiment
[Fig. 11 ] Diagnosis flowchart of the third embodiment.
System diagram showing a fourth embodiment of the present invention; FIG
[13] Ibid diagnosis flowchart of the fourth embodiment
System diagram showing a fifth embodiment of Figure 14 the present invention
[15] [Description of symbols] Diagnostic flowcharts of the same fifth embodiment
1 Engine 2 Intake Passage 3 Fuel Injection Valve 4 Control Unit 5 Exhaust Passage 6 O ₂ Sensor 7 Lean NOx Catalyst 8 NOx Adsorption Catalyst 9 NOx Sensor 10 Temperature Sensor 11 Upstream NOx Sensor

Claims (4)

In an engine provided with an NOx adsorption catalyst capable of adsorbing NOx in an exhaust passage when operating at a lean air-fuel ratio and reducing the adsorbed NOx during operation at a stoichiometric air-fuel ratio,
While providing a NOx sensor for detecting NOx in the exhaust downstream of the NOx adsorption catalyst and a temperature sensor for detecting the temperature of the NOx adsorption catalyst,
Means for detecting the time until the output value of the NOx sensor exceeds a preset slice level after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, and the accumulated time during which the catalyst temperature is in a high temperature state equal to or higher than a predetermined value When the time until the slice level is exceeded is less than a predetermined value and the accumulated time is greater than the predetermined value, it is determined that the NOx adsorption catalyst has deteriorated, and the time until the slice level is exceeded is a predetermined value. smaller and the integration time is in the case smaller than a predetermined value is provided and a NOx adsorption catalyst deterioration determining means for determining a temporary deterioration by the S-poisoning of said NOx trap catalyst diagnosis apparatus. The exhaust passage is provided with a lean NOx catalyst capable of adsorbing HC during operation at the stoichiometric air-fuel ratio and reducing NOx by the adsorbed HC during operation at the lean air-fuel ratio, and further on the downstream side at the lean air-fuel ratio. A NOx sensor for detecting NOx in exhaust gas downstream of the NOx adsorption catalyst in an engine including a NOx adsorption catalyst that adsorbs NOx during operation and can reduce the adsorbed NOx during operation at a theoretical air-fuel ratio; While providing a temperature sensor for detecting the temperature of the NOx adsorption catalyst,
Means for detecting the time until the output value of the NOx sensor exceeds a preset slice level after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, and the accumulated time during which the catalyst temperature is in a high temperature state equal to or higher than a predetermined value means for calculating the time to more than slice level if and the integration time less than a predetermined value is larger than a predetermined value is determined as the deterioration of the NOx adsorption catalyst, time Tokoro value to exceed the slice level When the accumulated time is smaller than the predetermined value , the lean NOx catalyst is determined based on the NOx adsorption catalyst deterioration determining means for determining temporary deterioration due to S poisoning of the NOx adsorption catalyst and the output value level of the NOx sensor. And a lean NOx catalyst deterioration determining means for determining deterioration of the fuel. An engine having a catalyst in the exhaust passage is provided with an upstream NOx sensor for detecting NOx in the exhaust upstream of the catalyst, while the output value of the upstream NOx sensor depends on the air-fuel ratio when operating at a lean air-fuel ratio. Air-fuel ratio shift determination that determines that the air-fuel ratio is richly shifted when greater than the set upper limit value and that the air-fuel ratio is lean-shifted when it is smaller than the lower limit value that is set according to the air-fuel ratio Diagnostic device provided with means. An upstream NOx sensor for detecting NOx in the exhaust gas is provided upstream of the catalyst, and an air-fuel ratio shift for determining an air-fuel ratio shift based on the level of the output value of the upstream NOx sensor when operating at a lean air-fuel ratio. 3. The diagnostic apparatus according to claim 1, further comprising a determination unit.

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