METHOD FOR OPERATING AN EXHAUST PURIFICATION DEVICE FOR A MOTOR VEHICLE AND CORRESPONDING EXHAUST PURIFICATION DEVICE

Description

[0001] The invention relates to a method for operating an exhaust gas purification device for a motor vehicle, wherein the exhaust gas purification device comprises a vehicle catalytic converter, a first lambda sensor arranged upstream of the vehicle catalytic converter, and a second lambda sensor arranged downstream of the vehicle catalytic converter, wherein a signal gradient is determined between a first signal step of a measured value from the first lambda sensor in a first direction and a second signal step of the measured value in a second direction opposite to the first direction, and a signal delay of a measured value from the second lambda sensor is determined after the second signal step, wherein the signal delay corresponds to the time period between the second signal step and a reaction of the measured value from the second lambda sensor to the second signal step. The invention further relates to an exhaust gas purification device for a motor vehicle.

[0002] For example, German patent application DE 10 2004 055 231 B3 is known from the prior art. This document describes a method for lambda control in an internal combustion engine with a catalytic converter in the exhaust system and at least one lambda sensor located inside the catalytic converter. With this arrangement of the upstream sensor, signal delays occur, which reduce the speed of the lambda controller. To compensate for this, the measurement signals from the first lambda sensor are applied to a lambda evaluation unit, which corrects the delay in the measurement signals, and the corrected lambda sensor signal is applied to a lambda control unit. Both lambda sensors are connected to a lambda control unit.

[0003] Furthermore, the publication DE 10 2016 213 767 A1 discloses a method for diagnosing an exhaust system of an internal combustion engine with at least one three-way catalytic converter, at least one four-way catalytic converter and at least one binary lambda sensor, in which a check of the functionality of the at least one binary lambda sensor and / or the at least one four-way catalytic converter is carried out on the basis of a lambda change when the internal combustion engine switches from a lean operation to a rich operation following overrun operation in order to clear out the at least one three-way catalytic converter.

[0004] Finally, US Publication 2016 / 0 160 778 A1 describes a diagnostic system for an internal combustion engine. The diagnostic system calculates a first characteristic curve of the change in the air-fuel ratio at the time the air-fuel ratio passes through a first air-fuel ratio range that is leaner than a stoichiometric air-fuel ratio, and a second characteristic curve of the change in the air-fuel ratio at the time the air-fuel ratio passes through a second air-fuel ratio range that includes a stoichiometric air-fuel ratio. The diagnostic system diagnoses a sensor deviation based on the first and second characteristic curves.

[0005] The object of the invention is to propose a method for operating an exhaust gas purification device for a motor vehicle which has advantages over known methods, in particular enabling a simple check of the functionality of the second lambda probe and / or the vehicle catalyst.

[0006] According to the invention, this is achieved by a method for operating an exhaust gas purification device for a motor vehicle with the features of claim 1. It is provided that a defect in the vehicle catalyst is detected if the signal gradient is less than a signal gradient threshold and the signal delay is less than a signal delay threshold.

[0007] Advantageous embodiments with appropriate further developments of the invention are specified in the dependent claims.

[0008] The method serves to operate the exhaust gas purification system. The exhaust gas purification system is, for example, a component of the motor vehicle, but can of course also exist separately. The exhaust gas purification system is preferably part of a drive system of the motor vehicle, which serves to propel the motor vehicle and thus to provide a drive torque directed towards propelling the motor vehicle. The drive system has at least one drive unit to generate the drive torque.

[0009] The drive unit, in particular the drive motor, generates exhaust gas during its operation, which is discharged towards the outside environment. The exhaust gas cleaning system is located between the drive motor and the outside environment. The exhaust gas cleaning system serves to clean the exhaust gas, i.e., to convert pollutants into less harmful substances. The exhaust gas generated by the drive motor is fed into the exhaust gas cleaning system and, after passing through it, is released into the outside environment.

[0010] Exhaust gas purification is carried out by the exhaust gas purification system using the vehicle's catalytic converter. The vehicle's catalytic converter can be, for example, a three-way catalytic converter, an oxidation catalytic converter, a storage catalytic converter (especially a NOx storage catalytic converter), or an SCR catalytic converter. A particulate filter can be integrated into the vehicle's catalytic converter. With respect to the exhaust gas flow direction through the exhaust gas purification system or the vehicle's catalytic converter, the first lambda sensor is located upstream of the vehicle's catalytic converter, while the second lambda sensor is located downstream of the vehicle's catalytic converter.

[0011] The lambda sensors are used to determine the air-fuel ratio λ. For this purpose, they measure, in particular, the residual oxygen content present in the exhaust gas. Each lambda sensor provides a corresponding measurement value. The first lambda sensor is preferably a wideband sensor and the second a narrowband sensor. In this case, the measurement value of the first lambda sensor is preferably expressed as the air-fuel ratio λ, and the measurement value of the second lambda sensor as a voltage.

[0012] The wideband lambda sensor is characterized by its ability to determine the air-fuel ratio over a broad range. For example, the wideband lambda sensor incorporates a pump cell and a Nernst cell, between which a measuring volume is located. This measuring volume is fluidically connected to an exhaust gas line via a diffusion channel running through the pump cell. The current flowing through the pump cell is adjusted to maintain an air-fuel ratio of λ = 1 within the measuring volume. This is achieved by using a measurement from the Nernst cell, which is adjusted to a value corresponding to the air-fuel ratio of λ = 1, typically by means of a control system.

[0013] In comparison, the transition lambda sensor has a simpler design and consists, for example, of only a Nernst cell. A transition lambda sensor's characteristic curve is marked by a large gradient in its measured value around a combustion air-fuel ratio of λ = 1. The gradient of the measured value is therefore significantly greater in the region around λ = 1 than outside this range.

[0014] For example, the reading from the first lambda sensor is used to control the lambda value of the engine or drive unit. The reading from the second lambda sensor, on the other hand, is used for trim control. Trim control is used to compensate for any deviations from the first lambda sensor, thus enabling more precise lambda control.

[0015] Since the second lambda sensor is essential for the accuracy of the lambda control system, it is necessary to diagnose faults in the second lambda sensor as quickly as possible in order to initiate appropriate countermeasures. For example, if the second lambda sensor is defective, the rated power of the drive system or drive unit is reduced. To diagnose the second lambda sensor, the signal gradient of the measured value from the second lambda sensor and the signal delay of the measured value are determined. This determination is performed as a function of time between the first and second signal steps of the measured value from the first lambda sensor. Thus, the signal gradient is determined temporally between the first and second signal steps, and the signal delay is determined temporally after the second signal step.

[0016] The signal gradient refers to the increase in the measured value of the second lambda sensor over time, whereas the signal delay describes the lag in the measured value of the second lambda sensor. The signal delay thus corresponds to the time interval between the second signal step and the reaction of the measured value of the second lambda sensor to this second signal step. The first and second signal steps refer to jumps in the measured value of the first lambda sensor, i.e., measured values ​​for which a gradient of the measured value, in particular an absolute value of the gradient, exceeds a gradient threshold. It is therefore initially irrelevant whether the measured value increases or decreases. What matters is only that the sign-corrected gradient exceeds the gradient threshold.

[0017] The signal gradient and signal delay are evaluated to diagnose the second lambda sensor. In principle, it could be possible to detect a defect in the second lambda sensor if the signal gradient is less than the signal gradient threshold. The signal gradient threshold is a value for the signal gradient, preferably determined empirically, and below which a defect in the second lambda sensor can be assumed. However, it should be noted that the measurement signal of the second lambda sensor does not directly follow the measurement signal of the first lambda sensor, but also depends on the vehicle's catalytic converter, and in particular its condition.

[0018] To avoid falsely indicating a defect in the second lambda sensor due to the influence of the vehicle's catalytic converter, the signal delay is used in addition to the signal gradient for diagnosing the second lambda sensor. Thus, if the signal gradient falls below the threshold value, a defect in the second lambda sensor is only detected if the signal delay is also greater than the signal delay threshold. The signal delay threshold is also preferably determined empirically and has a value below which it can be assumed that the second lambda sensor is defective, particularly if the signal gradient is also smaller than the signal gradient threshold.

[0019] To detect a defect in the second lambda sensor, it is not sufficient for the signal gradient to simply be smaller than the signal gradient threshold. Rather, the signal delay must also be greater than the signal delay threshold. In other words, if the signal gradient falls below the threshold, a defect in the second lambda sensor will only be detected if the signal delay is simultaneously greater than the signal delay threshold. This ensures a reliable diagnosis of the second lambda sensor.

[0020] Additionally or alternatively, the signal gradient and signal delay can be used to diagnose the vehicle's catalytic converter. As explained above, the reading from the second lambda sensor does not simply follow the reading from the first lambda sensor, but also depends on the condition of the vehicle's catalytic converter. If the signal gradient is lower than the signal gradient threshold and the signal delay is also lower than the signal delay threshold, a defect in the vehicle's catalytic converter will be detected. In other words, if the signal gradient falls below the signal gradient threshold, a defect in the vehicle's catalytic converter will only be detected if the signal delay is lower than the signal delay threshold.

[0021] Preferably, the diagnosis of the second lambda sensor and / or the vehicle's catalytic converter is performed in two stages. First, only the signal gradient is compared with the signal gradient threshold, and the signal delay is determined but initially disregarded. Only if the signal gradient is less than the signal gradient threshold is the signal delay compared with the signal delay threshold in a second step. If the signal delay is greater than the signal delay threshold, a defect in the second lambda sensor is detected. Additionally or alternatively, the functionality of the vehicle's catalytic converter can be detected. However, if the signal delay is less than the signal delay threshold, a defect in the vehicle's catalytic converter is detected. Additionally or alternatively, the functionality of the second lambda sensor can be detected.

[0022] The described procedure enables a particularly precise diagnosis of the second lambda sensor and / or the vehicle's catalytic converter. In particular, false positives for a second lambda sensor defect are effectively avoided by using not only one parameter, namely the signal gradient, for the diagnosis, but also an additional parameter, namely the signal delay. This procedure also allows – optionally – an assessment of the vehicle's catalytic converter's condition.

[0023] A further development of the invention provides that the largest gradient of the measured value from the second lambda sensor occurring between the first and second signal steps is used as the signal gradient. For example, the gradient of the measured value from the second lambda sensor between the first and second signal steps is determined continuously or periodically. The largest gradient occurring during this period is used as the signal gradient. Preferably, the signal gradient is sign-corrected, i.e., it corresponds to an absolute value of the largest gradient between the two signal steps. This achieves high accuracy in the diagnosis of the second lambda sensor.

[0024] A further development of the invention provides that the signal delay is defined as the time interval between the second signal step and a change in sign of the gradient of the measured value from the second lambda sensor. As already explained, the signal delay corresponds to the time interval after which the measured value from the second lambda sensor shows a reaction to the second signal step. The change in sign of the gradient of the measured value from the second lambda sensor serves as the criterion for the reaction. This change in sign occurs because the second signal step describes a deflection of the measured value from the first lambda sensor in a different direction than occurs during the first signal step. Thus, while the measured value from the first lambda sensor changes in the first direction during the first signal step, it changes in the second direction during the second signal step, which is opposite to the first direction.Accordingly, the signal delay can be determined with high accuracy based on the sign change.

[0025] A further development of the invention provides that the first signal step and / or the second signal step is detected when the measured value of the first lambda sensor changes from a value greater than or equal to one to a value less than or equal to one, or from a value less than or equal to one to a value greater than or equal to one, by exceeding a certain value difference. For the first or second signal step to occur, it is therefore always necessary that the measured value of the first lambda sensor changes by the certain value difference during the respective signal step, where the value difference is understood to be the difference between the measured value of the first lambda sensor immediately before the respective signal step and its measured value immediately after the respective signal step.

[0026] During the signal transition, the measured value changes, for example, from a value greater than one to a value of one or to a value less than one. It can also change from a value of one to a value less than one. The reverse is also true. A reading of one from the first lambda sensor corresponds to a stoichiometric air-fuel ratio. Similarly, a reading of greater than one from the first lambda sensor corresponds to an excess of air, and a value less than one to a deficiency of air in the exhaust gas.

[0027] The value difference must be, for example, at least 0.01, at least 0.02, at least 0.03, at least 0.04, or at least 0.05, with the latter two values ​​being preferred. It may also be provided that the value difference for detecting the first signal step is greater than the value difference for detecting the second signal step. For example, the former is greater than the latter by a factor of at least 1.5, at least 1.75, or at least 2.0. In this case, detection of the first signal step occurs, for example, only when the value difference is at least 0.05, at least 0.075, or at least 0.10, whereas detection of the second signal step occurs when the value difference is between 0.01 and 0.05 (or greater).

[0028] The first signal jump is only detected with particular priority if the reading from the first lambda sensor indicates a change from excess air to a lack of air. The second signal jump, for example, is only detected if a change from a lack of air to at least a stoichiometric air-fuel ratio or excess air is detected. This ensures a high degree of accuracy in the diagnosis of the second lambda sensor.

[0029] A further development of the invention provides that the second lambda sensor is detected as functioning when the signal gradient exceeds the threshold value. Thus, if the signal gradient is greater than the threshold value, it can be reliably assumed that the second lambda sensor is functioning. In this case, an evaluation of the signal delay is unnecessary. For example, the functionality of the second lambda sensor can be determined immediately after the second signal step and without determining the signal delay, namely when the signal gradient exceeds the threshold value. This enables a particularly rapid evaluation. For example, it is intended that the signal delay is only determined when the signal gradient falls below the threshold value.However, if the signal gradient exceeds the signal gradient threshold, the signal delay is not determined.

[0030] A further development of the invention provides that if the signal gradient falls below the threshold and the signal delay also falls below the threshold, the system detects that the second lambda sensor is functioning correctly. In other words, a defect in the second lambda sensor is only detected if, firstly, the signal gradient falls below the threshold and, secondly, the signal delay is greater than the threshold. If either of these conditions is not met, the system assumes that the second lambda sensor is functioning correctly. As explained above, depending on the signal gradient, the evaluation of the signal delay can even be omitted. This enables a rapid and accurate diagnosis of the second lambda sensor.

[0031] A further development of the invention provides that if the signal delay threshold is exceeded, the defect of the second lambda sensor is always detected. If the signal delay is evaluated, for example, as a function of the signal gradient value, the defect of the second lambda sensor is always detected if the signal delay is greater than the signal delay threshold. It is therefore irrelevant whether the signal delay is evaluated solely as a function of the signal gradient value or whether the signal delay is always determined independently of the signal gradient value. If the aforementioned criterion is met, the defect of the second lambda sensor is always detected.

[0032] A further development of the invention provides that, when the signal delay threshold is exceeded, the signal gradient is compared with the signal gradient threshold. If the signal gradient falls below a further signal gradient threshold, a defect in the vehicle catalytic converter is detected, and if it exceeds a further signal gradient threshold, the vehicle catalytic converter is detected as functioning correctly. This allows for the diagnosis of the vehicle catalytic converter in addition to the diagnosis of the second lambda sensor.

[0033] While exceeding the signal delay threshold always indicates a defect in the second lambda sensor, a definitive statement about the condition of the vehicle's catalytic converter can be made using the secondary signal gradient threshold. This secondary signal gradient threshold is lower than the primary signal gradient threshold. If the signal gradient falls below the primary signal gradient threshold, a defect in the vehicle's catalytic converter is suspected. For this reason, a more detailed analysis is performed using the secondary signal gradient threshold.

[0034] If the signal gradient is lower than the further signal gradient threshold, a defect in the vehicle's catalytic converter, in addition to a defect in the second lambda sensor, can be assumed with certainty. Conversely, if the signal gradient exceeds the further signal gradient threshold, i.e., if the signal gradient lies between the further signal gradient threshold and the signal gradient threshold, the vehicle's catalytic converter can be assumed to be functioning, or at least to a limited extent. The described procedure allows for the diagnosis not only of the second lambda sensor but also of the vehicle's catalytic converter.

[0035] A further development of the invention provides that the additional signal gradient threshold is chosen to be lower than the signal gradient threshold. This has already been mentioned above. For example, the additional signal gradient threshold is at most 75%, at most 60%, or at most 50% of the signal gradient threshold. Such a selection of the additional signal gradient threshold allows for a differentiated analysis of the functionality of the vehicle catalyst.

[0036] The invention further relates to an exhaust gas purification device for a motor vehicle, in particular for carrying out the method according to the description in this document, wherein the exhaust gas purification device comprises a vehicle catalyst, a first lambda sensor arranged upstream of the vehicle catalyst and a second lambda sensor arranged downstream of the vehicle catalyst, wherein the exhaust gas purification device is designed and configured to determine a signal gradient between a first signal jump of a measured value from the first lambda sensor in a first direction and a second signal jump of the measured value in a second direction opposite to the first direction, and a signal delay of a measured value from the second lambda sensor after the second signal jump.where the signal delay corresponds to the time period between the second signal jump and the reaction of the measured value of the second lambda sensor to the second signal jump.

[0037] The exhaust gas purification device is further designed and configured to detect a defect in the vehicle catalyst if the signal gradient is less than a signal gradient threshold and the signal delay is less than a signal delay threshold.

[0038] The advantages of such a design of the exhaust gas purification system and such a procedure have already been mentioned. Both the exhaust gas purification system and the method for operating it can be further developed as described in this document, and reference is made to these details.

[0039] The features and combinations of features described in the description, in particular those described in the subsequent figure description and / or shown in the figures, can be used not only in the combinations specified, but also in other combinations or individually, without departing from the scope of the invention. Thus, embodiments that are not explicitly shown or explained in the description and / or the figures, but which result from or can be derived from combinations of features in the explained embodiments, are also to be considered as being covered by the invention.

[0040] The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without limiting the invention. The drawing shows: Figure 1 is a schematic representation of a drive unit for a motor vehicle, and Figure 2 is two diagrams which illustrate a method for operating an exhaust gas purification device of the drive unit.

[0041] The Figure 1Figure 1 shows a schematic representation of a drive unit 1 for a motor vehicle. The drive unit 1 has an exhaust gas-generating drive unit 2, in the embodiment shown here an internal combustion engine. The exhaust gas generated by the drive unit 2 is fed to an exhaust gas purification unit 3, which, in addition to a vehicle catalyst 4, has a first lambda sensor 5 and a second lambda sensor 6. The first lambda sensor 5 is arranged upstream of the vehicle catalyst 4 with respect to the exhaust gas flow direction, and the second lambda sensor 6 is arranged downstream of the vehicle catalyst 4. A measured value from the first lambda sensor 5 is fed to a first controller 7, which performs lambda control of the drive unit 2. A measured value from the second lambda sensor 6, on the other hand, is fed to a second controller 8, which performs trim control, correcting deviations in the measured value of the first lambda sensor 5.

[0042] The Figure 2 Figure 1 shows two diagrams illustrating a method for operating the exhaust gas purification device 3 of the drive unit 1. The upper diagram shows the course of a measured value from the first lambda sensor 5 over time t. These measured values ​​are to be understood as the air-fuel ratio lambda, where λ = 1 corresponds to a stoichiometric air-fuel ratio. The lower diagram shows the course of measured values ​​from the second lambda sensor 6 over time t. These measured values ​​are to be understood as electrical voltages, where the voltage U0 is present at a stoichiometric air-fuel ratio.

[0043] As part of the procedure for operating the exhaust gas purification device 3, a diagnosis of the second lambda sensor 6 is performed. The drive unit 2 is operated in such a way that the measured values ​​of the first lambda sensor 5 exhibit several signal jumps over time t, in particular a first signal jump 9 and a second signal jump 10. The two signal jumps 9 and 10 are clearly recognizable in a curve 11 of the measured values ​​of the first lambda sensor 5 over time t. For example, it is intended to operate the drive unit 2 in such a way that, at time t 0, starting from a stoichiometric air-fuel ratio, an air-fuel ratio greater than one is set, in particular an air-fuel ratio of 1.05.

[0044] To generate the first signal step 9, at time t1 the drive unit 2 is controlled such that the air-fuel ratio drops below one, specifically to 0.95. The first signal step 9 thus comprises a change in the measured value of the first lambda sensor 5 by a difference of 0.1. To generate the second signal step 10 at time t2, the drive unit 2 is controlled such that the air-fuel ratio is adjusted from a value of less than one to a value of one. This value is subsequently maintained at least until time t3.

[0045] Depending on the state of the second lambda sensor 6 and the vehicle catalyst 4, the measured value of the second lambda sensor 6 exhibits different curves 12, 13, and 14. Curve 12 shows the measured values ​​over time t when both the second lambda sensor 6 and the vehicle catalyst 4 are functioning correctly. Curve 13 shows the measured values ​​of the second lambda sensor 6 over time t when the second lambda sensor 6 is defective but the vehicle catalyst 4 is functioning correctly. Curve 14, on the other hand, shows the measured value of the second lambda sensor 6 over time when the second lambda sensor 6 is functioning correctly, but this value differs from curve 12 due to the operating state of the vehicle catalyst 4.

[0046] To diagnose the second lambda sensor 6, a signal gradient 15, 16, or 17 is to be determined between signal jumps 9 and 10 for curves 12, 13, and 14. This is only indicated. If the respective signal gradient 15, 16, or 17 is greater than a signal gradient threshold, the second lambda sensor 6 is assumed to be functioning correctly. This condition applies to curve 12, so the functionality of the second lambda sensor 6 can be directly determined. For comparison, curve 13 is shown, for which the signal gradient 16 is smaller than the signal gradient threshold. In this case, a defect in the second lambda sensor 6 can be directly identified.

[0047] For curve 14, however, the signal gradient is smaller than the signal gradient threshold, even though the second lambda sensor 6 is fundamentally functional. Therefore, it is planned to determine a signal delay Δt, at least in cases where the signal gradient falls below the signal gradient threshold after the second signal step 10. The signal delay Δt describes the time after which the measured value of the second lambda sensor 6 reacts to the second signal step 10. It can be seen that for curves 12 and 14, the reaction occurs almost immediately, so that the corresponding signal delay is in each case below the signal delay threshold. For curve 13, however, the signal delay Δt is greater than the signal delay threshold. In this case, a defect in the second lambda sensor 6 can be detected.

[0048] Based on both the signal gradient and the signal delay, it is possible to reliably determine whether the second lambda sensor 6 is functional or defective. A defect in the second lambda sensor 6 is only detected if the signal gradient falls below the threshold value and the signal delay Δt is also greater than the threshold value. If this is the case, the second lambda sensor 6 is detected as functional even if the signal gradient is less than the threshold value. This procedure enables a reliable diagnosis of the vehicle's catalytic converter 4 and, optionally, the second lambda sensor 6. REFERENCE MARK LIST:

[0049] 1. Drive unit 2. Drive unit 3. Exhaust gas purification system 4. Vehicle catalytic converter 51. Lambda sensor 62. Lambda sensor 71. Controller 82. Controller 91. Signal step 102. Signal step 11. Curve 12. Curve 13. Curve 14. Curve 15. Signal gradient 16. Signal gradient 17. Signal gradient

Claims

1. Method for operating an exhaust gas cleaning device (1) for a motor vehicle, wherein the exhaust gas cleaning device (3) has a vehicle catalytic converter (4), a first lambda probe (5) arranged upstream of the vehicle catalytic converter (4), and a second lambda probe (6) arranged downstream of the vehicle catalytic converter (4), wherein a signal gradient (15, 16, 17) is determined chronologically between a first signal jump (9) of a measured value of the first lambda probe (5) and a second signal jump (10) of the measured value in a second direction opposite to the first direction, and a signal delay of a measured value of the second lambda probe (6) is determined after the second signal jump (10), wherein the signal delay corresponds to a period of time between the second signal jump (10) and a reaction of the measured value of the second lambda probe (6) to the second signal jump (10), characterized in that a defect in the vehicle catalytic converter (4) is identified if the signal gradient (15, 16, 17) is less than a signal gradient threshold and the signal delay is less than a signal delay threshold value.

2. Method according to claim 1, characterized in that a defect of the second lambda probe (6) is identified if the signal gradient (15, 16, 17) is less than the signal gradient threshold value and the signal delay is greater than the signal delay threshold value.

3. Method according to any one of the preceding claims, characterized in that the greatest gradient of the measured value of the second lambda probe (6) occurring between the first signal jump (9) and the second signal jump (10) is used as the signal gradient (15, 16, 17).

4. Method according to any one of the preceding claims, characterized in that a period of time between the second signal jump (10) and a sign change of the gradient of the measured value of the second lambda probe (6) is used as the signal delay.

5. Method according to any one of the preceding claims, characterized in that the first signal jump (9) and / or the second signal jump (10) is identified when the measured value of the first lambda probe (5) changes from a value of greater than or equal to one to a value less than or equal to one or from a value less than or equal to one to a value greater than or equal to one while exceeding a specific value difference.

6. Method according to any one of the preceding claims, characterized in that when the signal gradient (15, 16, 17) exceeds the signal gradient threshold value, a functional capability of the second lambda probe (6) is identified.

7. Method according to any one of the preceding claims, characterized in that when the signal gradient (15, 16, 17) falls below the signal gradient threshold value and in addition the signal delay falls below the signal delay threshold value, a functional capability of the second lambda probe (6) is identified.

8. Method according to claim 2, characterized in that when the signal delay exceeds the signal delay threshold value, the signal gradient (15, 16, 17) is compared to the signal gradient threshold value, wherein if the signal gradient (15, 16, 17) falls below a further signal gradient threshold value, the defect of the vehicle catalytic converter (4) is identified, and if the signal gradient exceeds the further signal gradient threshold value, a functional capability of the vehicle catalytic converter (4) is identified, wherein the further signal gradient is selected to be smaller than the signal gradient threshold.

9. Exhaust gas cleaning device (3) for a motor vehicle, in particular for carrying out the method as claimed in any one or more of the preceding claims, wherein the exhaust gas cleaning device (3) has a vehicle catalytic converter (4), a first lambda probe (5) arranged upstream of the vehicle catalytic converter (4), and a second lambda probe (6) arranged downstream of the vehicle catalytic converter (4), wherein the exhaust gas cleaning device (3) is provided and designed to determine a signal gradient (15, 16, 17) chronologically between a first signal jump (9) of a measured value of the first lambda probe (5) and a second signal jump (10) of the measured value in a second direction opposite to the first direction, and a signal delay of a measured value of the second lambda probe (6) after the second signal jump (10), wherein the signal delay corresponds to a period of time between the second signal jump (10) and a reaction of the measured value of the second lambda probe (6) to the second signal jump (10), characterized in that the exhaust gas cleaning device (3) is further provided and designed to identify a defect in the vehicle catalytic converter (4) the signal gradient (15, 16, 17) less than a signal gradient threshold and the signal delay is less than a signal delay threshold value.