Method for Diagnosing an Exhaust Gas Catalytic Converter and/or an Exhaust Gas Sensor of a Motor Vehicle Internal Combustion Engine

Abstract

A method for diagnosing an exhaust gas catalytic convertor and/or an exhaust gas sensor arranged downstream and/or upstream of the exhaust gas catalytic convertor in an exhaust gas system of a motor vehicle internal combustion engine. The internal combustion engine is operated with an air/fuel mixture having a substoichiometric air/fuel ratio. The internal combustion engine is switched off and an unfired towed operation with disabled fuel supply is performed in which air drawn in by the internal combustion engine is conveyed through the exhaust gas system to the exhaust gas catalytic convertor and exhaust gas sensor. The internal combustion engine is then operated with a substoichiometric air/fuel ratio and a signal of the exhaust gas sensor is evaluated with regard to a predefinable evaluation criterion, the signal being provided at least in a period of time between switch-off and the subsequent self-running of the internal combustion engine.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention relate to a method for diagnosing an exhaust gas catalytic convertor and/or an exhaust gas sensor arranged downstream and/or upstream of the exhaust gas catalytic convertor in an exhaust gas system of a motor vehicle internal combustion engine, with which a signal of the exhaust gas sensor is evaluated with regard to a predefinable evaluation criterion.

A large number of methods have already been proposed for diagnosis of three-way catalytic convertors with oxygen storage capability, with which the effect of a sudden change in the air/fuel ratio during operation of a motor vehicle internal combustion engine is evaluated with regard to various signal parameters of an exhaust gas probe arranged in particular behind the catalytic convertor.

For example, German Patent Document DE 10 2006 010 769 A1 discloses making a sudden change from engine operation with a leaner air/fuel ratio to engine operation with a rich air/fuel ratio, and establishing and assessing a differential area of signal profiles of a pre-catalytic convertor and post-catalytic convertor lambda probe. German Patent Document DE 10 2005 028 001 A1 discloses assessing a period of time required on the whole by a post-catalytic convertor probe to reach specific signal values when changing from lean operation to rich operation and back again.

Disadvantages with these methods include the fact that an exhaust gas purification capacity of the exhaust gas catalytic convertor is typically reduced both during the period of lean operation and during the period of rich operation and the fact that harmful substances enter the surrounding environment in unconverted form.

A sudden change to the air/fuel ratio is often also caused by transition into or from overrun operation. For example, German Patent Document DE 10 2004 061 603 A1 discloses evaluating a signal profile of a lambda probe during a return from overrun operation into traction operation with a predetermined air/fuel ratio. A drawback here is that overrun operation is often undesirable or cannot be represented.

Exemplary embodiments of the present invention provide a method for diagnosing an exhaust gas catalytic convertor and/or an exhaust gas sensor arranged downstream and/or upstream of the exhaust gas catalytic convertor in an exhaust gas system of a motor vehicle internal combustion engine, with which disadvantages encountered in the prior art are avoided, where possible.

With the method according to the invention, the internal combustion engine is operated with an air/fuel mixture having a substoichiometric air/fuel ratio (lambda), that is to say with a rich air/fuel mixture. Starting from the rich operation of the internal combustion engine, the engine is switched off, that is to say stopped. Once the internal combustion engine has been stopped, the internal combustion engine is operated in towed operation with disabled fuel supply, wherein air drawn in by the internal combustion engine is conveyed through the exhaust gas system to the exhaust gas catalytic convertor and to the exhaust gas sensor. Self-running of the internal combustion engine is then started with fired operation and a substoichiometric air/fuel ratio. Here, the self-running of the internal combustion engine is started after a predefined or predefinable period of time once the towed operation has finished, that is to say from a stoppage phase. To diagnose the exhaust gas catalytic convertor and/or the exhaust gas sensor, a signal of the exhaust gas sensor provided at least in a period from switch-off to the subsequent self-running of the internal combustion engine is evaluated with regard to a predefinable evaluation criterion. On the basis of the evaluation criterion, a decision is made as to whether the exhaust gas sensor or the exhaust gas catalytic convertor is in a correct state or whether there is faulty function or a defect.

As a result of the change from substoichiometric engine operation to towed operation with conveyance of air to the exhaust gas sensor, there is a quick, practically sudden change in atmosphere at the location of the exhaust gas sensor from reducing to oxidizing. A reverse change occurs during the subsequent transfer to the self-running of the internal combustion engine.

Since, in towed operation, the fuel supply to the internal combustion engine is disabled and merely air is pumped from the intake side of the internal combustion engine to the exhaust gas tract, this operating phase is emission-free. If, as is preferable, the internal combustion engine is operated before and after stoppage thereof with a merely slightly substoichiometric air/fuel ratio, the harmful emission is thus also low in these operating phases. On the whole, the diagnosis method according to the invention is therefore carried out with very low harmful emissions.

In addition, the diagnosis method is preferably be performed in conjunction with start-stop operation of the internal combustion engine or of the motor vehicle so that no difficulties associated with driving operation of the motor vehicle are associated with the setting of the double lambda change. In particular, it is not necessary to wait for or to induce overrun operation.

The towed operation according to the invention is carried out, for example, by driving the internal combustion engine by means of an electric starter, which is preferably a starter generator.

In an embodiment of the method, the method is carried out in a motor vehicle that has an electric motor drive. The motor vehicle is thus a hybrid vehicle, which can be driven both by internal combustion engine and by electric motor. Here, it is particularly preferable if the internal combustion engine is towed by the electric motor during the towed operation of the diagnosis method. The method according to the invention offers particular advantages with a hybrid vehicle, since an undesired overrun operation with a force fit between the engine output and transmission caused by the braking effect is omitted in this case.

In a further embodiment of the method, an exhaust gas sensor formed as a lambda probe with a jump characteristic (what is known as a binary sensor or binary probe) is diagnosed. This sensor is preferably arranged downstream of the exhaust gas catalytic convertor in the exhaust gas system, but may also be provided upstream of the exhaust gas catalytic convertor. A λ-dependant signal profile, which, with λ=1.0, has a jump point with an abruptly changing output signal, is typical for such a lambda probe. For diagnosis of the binary sensor, the signal profile thereof is preferably assessed by comparison with the profile of a signal supplied by a further lambda probe arranged before the exhaust gas catalytic convertor. In particular, this further lambda probe is formed as what is known as a broadband lambda probe with an output signal running continuously in accordance with lambda. Such a system design is widespread in automotive engineering, in particular in conjunction with an exhaust gas catalytic convertor formed as a three-way catalytic convertor. Here, it is particularly advantageous if, in a further embodiment of the invention, an exhaust gas sensor formed as a continuous lambda probe and arranged upstream of the exhaust gas catalytic convertor in the exhaust gas system is additionally or alternatively diagnosed. Of course, diagnosis of a broadband lambda probe arranged downstream of the exhaust gas catalytic convertor in the exhaust gas system is also possible. The exhaust gas sensor formed as a binary sensor and arranged downstream of the exhaust gas catalytic convertor, the exhaust gas sensor formed as a broadband probe and arranged upstream of the exhaust gas catalytic convertor, and/or the exhaust gas catalytic convertor can also be diagnosed by comparative assessment with a calculated or stored reference signal profile or lambda profile.

In a further embodiment of the method, an exhaust gas catalytic convertor capable of storing oxygen is diagnosed. This exhaust gas catalytic convertor is preferably formed as an oxidation catalytic convertor or as a three-way catalytic convertor with oxygen storage capability. Here, the signal profile of a downstream exhaust gas sensor is preferably evaluated with regard to an evaluation criterion that can be influenced by the extent of the oxygen storage capability.

In a further embodiment of the method, the internal combustion engine is operated with an air/fuel mixture having a lambda value of approximately 0.98 in method step ‘a’, that is to say with operation of the internal combustion engine with an air/fuel mixture having a substoichiometric air/fuel ratio before the internal combustion engine is switched off. Operation with very low harmful emissions is thus made possible.

In a further embodiment of the method, the internal combustion engine is supplied with an air/fuel mixture acting in a reducing manner and having a lambda value in the range from approximately 0.98 to approximately 0.90 in method step ‘d’, that is to say with self-running of the internal combustion engine subsequent to the towed phase. The emission of harmful substances is therefore also low in this operating phase.

In a further embodiment of the method, the signal of the exhaust gas sensor is evaluated with regard to at least one of the following evaluation criteria: signal gradient, dead time, settling time and symmetry. Here, the signal gradient is defined by a rising or falling gradient of the signal with the signal parameter characterizing lambda change. The dead time is given by a steady-state time of the signal after lambda change. The settling time is characterized by the time needed between the start of a signal change and the moment at which a specific, in particular approximately stable end value is reached in conjunction with a lambda change. In the case of the evaluation with regard to symmetry, the signals with rising and/or falling lambda change are compared.

Further advantages, features and details of the invention will emerge from the following description of preferred exemplary embodiments and with reference to the drawings. The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned hereinafter in the descriptions of the figures and/or shown alone in the figures cannot only be used in the specified combination, but can also be used in other combinations or in isolation without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

In the drawings:

FIG. 1 shows a schematic illustration of a motor vehicle/internal combustion engine arrangement with exhaust gas system for carrying out the method according to the invention,

FIG. 2 shows graphs with signal profiles and progressions over time of operating parameters of the arrangement illustrated in FIG. 1, and

FIG. 3 shows a diagram with exemplary signal profiles over time of a reference signal R of an assumed input variable and an assumed response signal A as an exemplary response of an arbitrary sensor.

DETAILED DESCRIPTION

The embodiment of the system illustrated schematically below in FIG. 1 is to be understood as an advantageous embodiment considered to be merely exemplary and in no way limiting and shows an internal combustion engine 1 preferably formed as a spark ignition engine for driving a motor vehicle (not illustrated), which obtains its combustion air via an intake air line 2 and obtains fuel via a fuel supply. The fuel is preferably directly injected via injection valves in such a way that an air/fuel mixture to be burned is formed in the combustion chambers of the engine 1, which is not illustrated in greater detail.

Combustion exhaust gases produced during combustion of the air/fuel mixture are fed via an exhaust gas system 3 to an exhaust gas catalytic convertor 4 for catalytic exhaust gas purification. The exhaust gas catalytic convertor 4 can be a three-way catalytic convertor with oxygen storage capability that is part of an exhaust gas purification system that may contain further components used for exhaust gas purification, such as a downstream nitrogen oxide storage catalytic convertor and/or an SCR catalytic convertor, which is not illustrated separately for reasons of clarity.

A first exhaust gas sensor 6 is arranged in the exhaust gas system 3 on the input side of the exhaust gas catalytic convertor 4. A second exhaust gas sensor 5 is arranged on the output side of the exhaust gas catalytic convertor 4. Furthermore, a temperature sensor 9 for measuring an exhaust gas temperature is provided upstream of the exhaust gas catalytic convertor 4. Without limiting the generality, it is assumed hereinafter that the exhaust gas sensors 5, 6 are lambda probes, which emit an output signal correlating with an air/fuel ratio λ or an oxygen partial pressure.

In the present case, it is assumed that the first lambda probe 6 is a continuously operating lambda probe. These types of probes are also referred to as broadband probes or LSU probes, which can emit an output signal changing constantly in accordance with the lambda value. The second lambda probe 5 is formed as a binary sensor in the present case. These types of probes are also referred to as 2-point probe or binary probe, which can emit an output signal having a jump point at a lambda value of 1.0. A person skilled in the art is familiar with the operating principle of exhaust gas sensors of this type and with the typical lambda-dependant output signal profiles thereof and these therefore will not be described here in greater detail. It goes without saying that other probe types or sensor types that generate an output signal changing in accordance with a lambda value can also be used. For example, a nitrogen oxide sensor can be used instead of the first and/or second lambda probe 5, 6.

The lambda probes 5, 6 and the temperature sensor 9 are connected via signal lines 8 to an electronic control unit 7. The control unit 7 is further connected via one or more further data lines 10 to the engine 1 and can control the operation of the engine in accordance with the signals of the connected probes or sensors 5, 6, 9. Here, the control unit 7 also receives information regarding relevant state variables of the internal combustion engine 1 and of the exhaust gas purification system, such as rotational speed, temperatures, and pressures of corresponding sensors or transducers, for example an air mass measuring device (not illustrated) in the intake air line 2, and may also emit control signals as setting variables to actuators, such as an AGR valve, an exhaust gas turbocharger (not illustrated) and further operating units. The control unit 7 is also able to adjust an injection of fuel as required or in another predefinable manner. For this purpose, the control unit 7 may consult stored characteristic maps or calculation and/or control routines. To carry out these functions, the control unit 7 can communicate via the data lines 8, 10, which are illustrated here merely by way of example, are connected to the respective components and can be formed as unidirectional or bidirectional signal or control lines.

In the present case, an electric motor (not illustrated) is also provided, which can tow the internal combustion engine 1. The electric motor can be formed as a conventional starter or as what is known as a starter generator. In a particularly preferred embodiment, the electric motor is formed and is incorporated into the drive system of the motor vehicle (likewise not illustrated) such that it can drive the vehicle together with the internal combustion engine or else alone, at least temporarily. For the vehicle preferably equipped with what is known as a hybrid drive, start-stop operation is preferably provided, with which operation of the internal combustion engine 1 is interrupted where necessary, in particular during stop phases without movement.

The method according to the invention will be explained in greater detail hereinafter on the basis of the graphs illustrated in FIG. 2.

In FIG. 2, signal profiles and progressions over time of various operating variables are illustrated in graphs I, II, III and IV with a common time axis t. It is assumed in accordance with the conditions shown in graph II by the trail 23 that a corresponding vehicle is operated with decreasing vehicle speed v until stoppage, which is maintained over the further course of the time section considered here. Drive by means of internal combustion engine initially occurs as far as the moment in time t₁, at which the fuel supply to the internal combustion engine 1 is switched off at low or negligible speed v. The control signal K_off responsible for switching off the internal combustion engine 1 or the fuel supply is illustrated in graph III by the trail 24.

A short time after switching off the fuel supply at the moment in time t₁, that is to say one second or a few seconds later, the electric motor of the vehicle is switched on at the moment in time t₂, which is illustrated by the profile of the trail 25 illustrated in graph IV for a signal EM_on responsible therefore. The electric motor is operated here in such a way that the internal combustion engine 1 is towed and air is accordingly conveyed via the intake line 2 into the exhaust gas system 3. In this case, the fuel supply continues to remain disabled, as can be inferred from the trail 24 in graph III. The towed operation of the internal combustion engine 1 preferably occurs at a predefined rotational speed, for example idling rotational speed, and in an unthrottled manner with regard to the air feed. The towed operation of the internal combustion engine 1 is maintained starting at the moment in time t₂ for a specific period of time from approximately 10 s to 30 s until the moment in time t₅. Here, the moment in time t₅, at which towed operation is terminated and the electric motor is switched off, is preferably determined by reaching a predefined signal value of the second lambda probe 5.

Once a further period of time from approximately 30 s to 120 s has elapsed, self-running of the internal combustion engine 1 with fired operation is induced at a moment in time t₆ as a result of brief tow of the internal combustion engine 1 by means of the electric motor and by re-establishing the fuel supply. The internal combustion engine 1 preferably runs in this instance with an idling rotational speed with the motor vehicle stopped.

The signal profiles of the lambda probes 5, 6 explained in greater detail hereinafter on the basis of graph I are considered or evaluated in conjunction with the above-described approach for diagnosis of the first lambda probe 6, the second lambda probe 5, and/or the exhaust gas catalytic convertor 4. Here, the trail 20 shown by a dashed line reproduces the output signal of the first lambda probe 6, formed in the present case as a broadband probe, for the values of which the left-hand ordinate λ_LSU is relevant. The output signal of the second lambda probe 5, which in the present case is a binary sensor, is reproduced by the trail 21, for which the right-hand ordinate U_LSF is relevant.

Until the moment in time t₁ of switch-off, the internal combustion engine 1 is operated in a fired manner with an air/fuel ratio with low fuel excess corresponding to a lambda value of approximately 0.98. Corresponding output signals 20, 21 that are approximately stable over time are therefore initially emitted by the lambda probes 5, 6. With the vehicle speed v tending toward zero, the signal 20 of the first lambda probe 6 initially shows a slow, increasingly steeper rise for the air/fuel ratio λ_LSU with switch-off of the fuel supply at the moment in time t₁ or after switch-on of the electric motor at the moment in time t₂. This rise is a response to a decreasing or ending exhaust gas delivery and a subsequent air supply into the exhaust gas system 3 caused by the towed operation by means of the electric motor, which ideally leads at the location of the first lambda probe 6 to a sudden rise of the lambda value from λ≈0.98 to λ=∞.

Similarly, but due to the curve characteristic complimentary to the rise of the signal 20 of the first lambda probe 6, the signal 21 of the second lambda probe 5 falls. Here, a time delay is noticeable, which is normally caused predominantly by storing the oxygen conveyed with the air in the exhaust gas catalytic convertor 4, which will be discussed in greater detail further below.

Correspondingly, but in the reverse direction, a signal profile of the lambda probes 5, 6 is to be observed if, starting from a stopped internal combustion and electric motor and air-filled exhaust gas system 3 at the time t₆, the internal combustion engine 1 is brought to a self-running state with a substoichiometric air/fuel ratio corresponding to a lambda value of approximately λ=0.90 to approximately λ=0.98.

As a result of gas transit times and in particular influenced by the functional quality of the lambda probes 5, 6, the respective signal rises and signal falls for λ_LSU and U_LSF respectively are more or less distorted compared to the respective, ideally at least approximately rectangular or abrupt lambda jump at the moment in time t₁ and t₆ respectively. This is utilized in accordance with the invention for diagnosis of the first lambda probe 6 and/or of the second lambda probe 5, as will be explained in greater detail further below.

Evaluation criteria derivable from the signal profile of the lambda sensors 5, 6 will be explained hereinafter with reference to FIG. 3. In the graph in FIG. 3, merely exemplary signal profiles over time of a reference signal R of an assumed input variable and an assumed response signal A as an exemplary response of an arbitrary sensor sensitive to the input variable are plotted. In the present case, the reference signal R is formed as a rectangle jump with a flank rising abruptly from zero to 100 and falling abruptly in a reverse direction.

The response signal A is distorted compared to the reference signal R. On the one hand, a rise at the moment in time t_u occurs later than the rise at the moment in time to of the reference signal R. The reaction time t_u−t₀ until the response signal A achieves a significant rise to a predefined value W₁ can be used as a delay or dead time as an evaluation criterion for the functional capability or quality of a respective sensor. If the dead time t_u−t₀ exceeds a predefined or predefinable value, a faulty sensor can thus be diagnosed. Furthermore, the response signal A rises with a reduced gradient compared to the reference signal R. For example, the maximum rise gradient can be used as an evaluation criterion for the quality or functional capability of the respective sensor, said maximum rise gradient being given by the slope of the tangent T at the steepest point of the response signal A. It can also be seen that the response signal A has only approached the end value of the reference signal, apart from a predefined slight difference, at a moment in time t_o. The corresponding settling time or response time t_o−t₀ until the response signal A reaches a predefined maximum value W₂ constitutes a further preferred evaluation criterion for functional capability. If the settling time t_o−t₀ exceeds a predefined or predefinable value, a faulty sensor can thus be diagnosed.

It goes without saying that further evaluation criteria based on a signal analysis can be used where necessary. For example, a frequency analysis or spectral analysis, for example by means of Fourier transformation, can be provided for the output signal A, or the output signal can be subject to mathematical signal filtration, from which evaluation criteria are obtained.

The evaluation criteria of dead time, signal gradient and settling time and possibly further evaluation criteria are preferably also applied analogously for the response signal A to the falling flank of the reference signal. The values relevant for diagnosis are not plotted separately in the graph in FIG. 3, but are given by analogous application of the explained approach mentioned for the rising flank. Symmetry relationships can be established from a comparison of evaluation criteria, applied to the response to the rising and falling flank, in particular with regard to dead time, signal gradient and settling time, as further evaluation criteria for sensor diagnosis. Should the dead time, signal gradient and settling time and possibly further signal variables of the output signal A for the rising and falling flank exceed predefinable maximum deviations, a reduced function or sensor error is diagnosed.

In accordance with the invention, the lambda rise actually occurring or a lambda rise coming as close as possible to the actual lambda rise as a result of the transition from internal combustion engine operation to towed operation is preferably used in practice, in application analogous to the above-explained approach, as a reference signal with regard to the response signals of the lambda probes 5, 6 for sensor diagnosis. The actual lambda fall or a lambda fall coming as close as possible to the actual lambda fall during the subsequent transfer from stoppage of the internal combustion engine 1 and electric motor to self-running of the internal combustion engine 1 is preferably also used as a reference signal for the response signals of the lambda probes 5, 6. Here, the respective actual lambda profile can be established mathematically on the basis of known operating variables, such as gas transit time; air-fuel ratio λ of the air:fuel mixture. The reference signal corresponding to the actual lambda profile at the location of the lambda probes 5, 6 may also be provided in the form of a characteristic curve stored in accordance with the values. For the signal of the second lambda probe 5, the signal of the first lambda probe 6 can also be used as a reference signal in order to obtain quantitative variables of the aforementioned evaluation criteria.

The signal of the second lambda probe 5 is preferably evaluated for diagnosis of the exhaust gas catalytic convertor 4. In particular, the dead time or delay time of the output signal of the second lambda probe 5 compared to a sudden lambda change caused by the approach according to the invention is used as a basis for a corresponding evaluation criterion. Reference will be made hereinafter again to graph I in FIG. 2 in order to explain the preferred approach.

As can be seen from graph I and as discussed further above, the output signal 21 of the second lambda probe 5 has a more or less noticeable delay compared to the lambda jump caused by transfer to towed operation at the moment in time t₁. The temporal delay is predominantly caused by storage of the oxygen conveyed with the air in the exhaust gas catalytic convertor 4. The storage of oxygen in the exhaust gas catalytic convertor 4 means that the sudden lambda increase caused by the transfer to towed operation and provided at the input of the exhaust gas catalytic convertor 4 may initially have no effect on the output side of the exhaust gas catalytic convertor 4. Here, the extent of the delay, that is to say the period of time until the lambda increase provided on the input side of the exhaust gas catalytic convertor 4 penetrates the output side of the exhaust gas catalytic convertor 4, is a measure for the oxygen storage capability of the exhaust gas catalytic convertor 4. However, an oxygen storage capability that is reduced compared to a new exhaust gas catalytic convertor 4 impairs the function of the exhaust gas catalytic convertor 4. For this reason, the oxygen storage capability is a quality criterion or a measure for the functional capability for the exhaust gas catalytic convertor 4.

A measure for the oxygen storage capability of the exhaust gas catalytic convertor 4 is preferably established as follows from the dead time or reaction time of the signal 21 of the second lambda sensor 5 compared to the sudden lambda rise caused by transfer to towed operation. The output signal 21 of the second exhaust gas sensor 5 is integrated starting at the moment in time t₃, at which the signal 20 of the first exhaust gas sensor 6 reaches a lambda value of λ_LSU=1.0. The integration is preferably terminated at the moment in time t₄, at which the signal 21 of the second exhaust gas sensor 5 falls below a predefinable value for U_LSF. The integral thus obtained is illustrated schematically in graph I by the trail 22. The obtained integral value is compared with a predefined or predefinable reference value for diagnosis of the exhaust gas catalytic convertor 4. Here, the reference value can be predefined, for example, in accordance with airflow rate conveyed in towed operation. A temperature dependence can additionally be taken into account, wherein the signal of the temperature sensor 9 can be used to establish the temperature. If the integral value falls below the reference value, the exhaust gas catalytic convertor 4 is thus to be classified as inadmissibly aged and a corresponding error message is thus to be output.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

Claims

1-14. (canceled)

15. A method for diagnosing an exhaust gas catalytic convertor or an exhaust gas sensor, which is arranged downstream or upstream of the exhaust gas catalytic convertor, in an exhaust gas system of a motor vehicle internal combustion engine, the method comprising:

a. operating the internal combustion engine with an air/fuel mixture having a substoichiometric air/fuel ratio;

b. switching the internal combustion engine off;

c. once the internal combustion engine has been stopped, carrying out unfired towed operation with a disabled fuel supply, wherein air drawn in by the internal combustion engine is conveyed through the exhaust gas system to the exhaust gas catalytic convertor and the exhaust gas sensor;

d. starting self-running of the internal combustion engine with fired operation and a substoichiometric air/fuel ratio; and

e. evaluating a signal of the exhaust gas sensor with regard to a predefinable evaluation criterion, wherein the signal is provided at least in a period of time between switch-off and the subsequent self-running of the internal combustion engine.

16. The method according to claim 15, wherein the method is carried out in a motor vehicle having an electric motor drive.

17. The method according to claim 15, wherein the exhaust gas sensor is a lambda probe with jump characteristic arranged downstream of the exhaust gas catalytic convertor in the exhaust gas system, and wherein the exhaust gas sensor is diagnosed.

18. The method The method according to claim 15, wherein the exhaust gas sensor is a continuous lambda probe arranged upstream of the exhaust gas catalytic convertor in the exhaust gas system, and wherein the exhaust gas sensor is diagnosed.

19. The method according to claim 15, wherein an exhaust gas catalytic convertor is configured with oxygen storage capability and the exhaust gas catalytic converter is diagnosed.

20. The method according to claim 15, wherein the internal combustion engine is operated with an air/fuel mixture having a lambda value of approximately 0.98 in method step a.

21. The method according to claim 15, wherein the internal combustion engine is supplied with an air/fuel mixture having a lambda value in a range from approximately 0.98 to approximately 0.90 in method step d.

22. The method according to claim 15, wherein the signal of the exhaust gas sensor is evaluated with regard to at least one of the following evaluation criteria: signal gradient, dead time, settling time and symmetry.