Abstract:
A first voltage (V 1 ) is detected between an auxiliary pump electrode and a reference electrode of an exhaust gas sensor, and a target diagnosis value (SDIAG) is determined as a function of the detected first voltage (V 1 ). A measured current (Im) is detected between a measurement electrode and a second main pump electrode of the exhaust gas sensor, said measured current being set up as the pump current by regulating a second voltage between the measurement electrode and the reference electrode to a pre-defined voltage. An actual diagnosis value (IDIAG) is determined as a function of the detected measured current (Im). An error (ERR) of the exhaust gas sensor is recognized depending on the target diagnosis value (SDIAG) and the actual diagnosis value (IDIAG).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a U.S. national stage of application No. PCT/EP2009/060673, filed on Aug. 18, 2009. Priority is claimed on German Application No. 10 2008 038 224.8, filed Aug. 18, 2008 the content of which is incorporated here by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1.Field of the Invention 
         [0003]    The invention relates to a method and to a device for testing an exhaust gas sensor and, in particular, an NOx sensor for motor vehicles with a diesel internal combustion engine. 
         [0004]    2. Description of the Prior Art 
         [0005]    Due to the increasingly strict legal requirements in terms of permissible emissions of pollutants in motor vehicles in which internal combustion engines are arranged there is a need to keep the emissions of pollutants during operation of the internal combustion engine as low as possible. This can be achieved, for example, by reducing the emissions of pollutants which are produced during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. On the other hand, exhaust gas post-treatment systems are used in internal combustion engines, said exhaust gas post-treatment systems converting the emissions of pollutants which are produced during the combustion process of the air/fuel mixture in the respective cylinder into harmless substances. For this purpose, catalytic converters are used which convert carbon monoxide, hydrocarbons and nitrogen oxides into harmless substances. Both the aimed-at influencing of the production of desired way over a long service life and faults are reliably detected. 
         [0006]    The manual “Handbuch Verbrennungsmotor” [Internal Combustion Engine Manual], published by Richard van Basshuysen/Fred Schafer, 2 nd  Edition, June 2002, the emissions of pollutants during the combustion and the conversion of the components of the pollutants with a high degree of efficiency by means of an exhaust gas catalytic converter require a very precisely set air/fuel ratio in the respective cylinder. NOx sensors are used to determine the nitrogen oxide content in the exhaust gas. In this context it is necessary to ensure that the components of the exhaust gas post-treatment system also function in the 
         [0007]    Friedrich Vieweg &amp; Sohn Verlagsgesellschaft mbH Braunschweig/Wiesbaden, pages 589 to 590, discloses an NOx sensor based on a ZrO 2  ceramic, which NOx sensor has two chambers. In the first chamber, a constant partial pressure of the oxygen contained in the exhaust gas is brought about by applying a pump current. The pump current is inverted proportional to the air/fuel ratio. In the second chamber, the nitrogen oxide contained in the exhaust gas is decomposed by applying a further current. This current is proportional to the nitrogen oxide content in the exhaust gas and forms the measuring signal of the NOx sensor. 
         [0008]    Document DE 199 07 947 B4 discloses a circuit for an NOx measuring pickup which has a first measuring cell and a second measuring cell which is connected to the first measuring cell. The measuring cells are located in a solid electrolyte. The circuit has a first circuit arrangement which sets a different oxygen concentration in the first measuring cell than in the gas to be measured by tapping a first Nernst voltage which serves as a first reference variable. A second circuit arrangement sets a different oxygen concentration in the second measuring cell than in the first measuring cell by tapping a second Nernst voltage which serves as a second reference variable. A third circuit arrangement drives a pump current composed of oxygen ions which originate from NOx out of the second measuring cell by tapping a third Nernst voltage which serves as a third reference variable. 
         [0009]    The exhaust gas sensor may be contaminated due to components in the exhaust gas. This may make it necessary to carry out diagnostics of the exhaust gas sensor during the ongoing operation of the internal combustion engine. 
       SUMMARY OF THE INVENTION 
       [0010]    The object of the invention is to provide a method and a device for testing an exhaust gas sensor which permit reliable detection of faults in the exhaust gas sensor. 
         [0011]    The invention is defined by a method and a corresponding device for testing an exhaust gas sensor. The exhaust gas sensor has a first chamber and a second chamber which are separated from one another by a diffusion barrier. The exhaust gas sensor also has a further diffusion barrier which forms a gas inlet of the first chamber. The first chamber has a first main pump electrode, and the second chamber has a measuring electrode and an auxiliary pump electrode. The exhaust gas sensor also has a second main pump electrode and a reference electrode outside the first and second chambers. A first voltage is sensed between the auxiliary pump electrode and the reference electrode. A set point diagnostic value is determined as a function of the sensed first voltage. A measuring current is sensed between the measuring electrode and the second main pump electrode. The measuring current is set as a pump current by adjusting a second voltage between the measuring electrode and the reference electrode to a predefined voltage. An actual diagnostic value is determined as a function of the sensed measuring current. A fault in the exhaust gas sensor is detected as a function of the set point diagnostic value and the actual diagnostic value. 
         [0012]    The advantage is that faults in the exhaust gas sensor, in particular contamination of the exhaust gas sensor and/or detachment of a protective layer from the measuring electrode, can be reliably detected. In addition, the testing of the exhaust gas sensor can advantageously also be reliably carried out in diesel internal combustion engines. 
         [0013]    The exhaust gas sensor is embodied, in particular, as an NOx sensor. However, the exhaust gas sensor can likewise be designed to sense one or more other components of the exhaust gas. The first and second voltages are, in particular, Nernst voltages. The first voltage is, in particular, representative of an oxygen content in the second chamber. In particular, for this purpose a predefined first pump current is set between the auxiliary pump electrode and the main pump electrode, and the resulting first voltage is then dependent on the oxygen content in the second chamber. 
         [0014]    As a result of the adjustment of the second voltage to the predefined voltage, essentially a stoichiometric gas mixture is formed in the surroundings of the measuring electrode, that is to say the oxygen from the surroundings of the measuring electrode is pumped away by the measuring current in such a way that the surroundings of the measuring electrode are essentially free of oxygen. The predefined voltage is, for example, approximately between 400 and 450 millivolts. However, the predefined voltage can also have a different value. 
         [0015]    The testing of the exhaust gas sensor is, in particular, carried out when at least one predefined diagnostic condition applies. The at least one predefined diagnostic condition can, in particular, comprise a predefined operating state, for example overrun conditions, or generally an operating state with an essentially nonvariable load and as a result an essentially nonvariable and predefined oxygen content in the exhaust gas. The at least one predefined diagnostic condition can also comprise, in particular, a low NOx measured value of the exhaust gas sensor which is below a predefined NOx threshold value. 
         [0016]    In one advantageous refinement, contamination of the exhaust gas sensor is detected as a fault if an oxygen concentration which is assigned to the actual diagnostic value in the second chamber is lower by at least a predefined first absolute value or factor than an oxygen concentration which is assigned to the set point diagnostic value in the second chamber. In particular, the contamination of the exhaust gas sensor is detected as a fault if the actual diagnostic value is smaller than the set point diagnostic value by at least a predefined lower threshold value. This has the advantage that the contamination of the exhaust gas sensor can be easily and reliably detected. 
         [0017]    In a further embodiment, detachment of a protective layer from the measuring electrode is detected as a fault if an oxygen concentration which is assigned to the actual diagnostic value in the second chamber is larger by at least a predefined second absolute value or factor than an oxygen concentration which is assigned to the set point diagnostic value in the second chamber. In particular, the detachment of the protective layer is detected as a fault if the actual diagnostic value is larger than the set point diagnostic value by at least a predefined upper threshold value. This has the advantage that the detachment of the protective layer can be easily and reliably detected. 
         [0018]    In a further embodiment, the set point diagnostic value is determined as a function of multiplication of the first voltage by a predefined factor and addition of a predefined shift value. The predefined factor and the predefined shift value are, in particular, predefined individually for the exhaust gas sensor or for a type or a design of the exhaust gas sensor and are determined, for example, as a function of calibration of the exhaust gas sensor, which calibration is carried out, in particular, at the premises of a manufacturer of the exhaust gas sensor within the scope of the manufacture of the exhaust gas sensor. The advantage is that this is easy. In addition, the set point diagnostic value can be determined reliably in this way. 
         [0019]    In a further embodiment, a further pump current is sensed between the first main pump electrode and the second main pump electrode. The set point diagnostic value is determined as a function of the sensed further pump current. The advantage is that the set point diagnostic value can be particularly reliably determined in this way. In addition, the testing of the exhaust gas sensor can be carried out even if the current oxygen content in the exhaust gas is unknown. The testing of the exhaust gas sensor can therefore be reliably carried out at different respective prevailing exhaust gas lambda values and can be carried out, in particular, at all prevailing exhaust gas lambda values. As a result, the testing of the exhaust gas sensor is also particularly suitable for diesel internal combustion engines in which exhaust gas lambda values can differ significantly from one, and under certain circumstances fluctuate to a high degree. The further pump current is preferably representative of an oxygen concentration in the exhaust gas and is, in particular, proportional to the oxygen concentration in the exhaust gas. 
         [0020]    In this context it is advantageous if the set point diagnostic value is determined as a function of multiplication of the first voltage by a predefined factor and addition of a predefined shift value. The predefined factor and/or the predefined shift value are selected as a function of the sensed further pump current. The predefined factor and the predefined shift value are, in particular, predefined individually for the exhaust gas sensor or for a type or a design of the exhaust gas sensor, for a respectively associated value or value range of the further voltage. The predefined factor, the predefined shift value and/or the respectively associated value or value range of the further pump current are determined, for example, as a function of calibration of the exhaust gas sensor. The calibration is carried out, for example, at the premises of a manufacturer of the exhaust gas sensor within the scope of the manufacture of the exhaust gas sensor. The advantage is that this is very easy and the diagnosis of the exhaust gas sensor can be carried out reliably in this way. The set point diagnostic value can be determined particularly reliably. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Exemplary embodiments of the invention are explained below with reference to the schematic drawings, in which: 
           [0022]      FIG. 1  shows an internal combustion engine with a control device, 
           [0023]      FIG. 2  shows a cross section through an exhaust gas sensor and a measuring circuit, 
           [0024]      FIG. 3  shows a diagram with diagnostic values which are plotted against a first voltage, and 
           [0025]      FIG. 4  shows a flowchart. 
       
    
    
       [0026]    Elements with the same design or function are provided with the same reference symbols in all the figures. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]      FIG. 1  shows an internal combustion engine having an intake section  10 , an engine block  12 , a cylinder head  13  and an exhaust section  14 . The intake section  10  preferably comprises a throttle valve  15 , a collector  16  and an intake manifold  17 . The intake manifold  17  is extended to a cylinder Z 1  in the inlet duct into a combustion chamber  26  of the engine block  12 . The engine block  12  comprises a crankshaft  18  which is coupled via a connecting rod  20  to a piston  21  of the cylinder Z 1 . 
         [0028]    The cylinder head  13  comprises a valve drive with a gas inlet valve  22  and a gas outlet valve  24 . The cylinder head  13  also comprises an injection valve  28 , and a spark plug  30  if the internal combustion engine is embodied, for example, as a petrol internal combustion engine. Alternatively, the injection valve  28  can also be arranged in the intake manifold  17 . 
         [0029]    An exhaust gas catalytic converter  32  is arranged in the exhaust section  14 . In addition, a catalytic converter  34  for reducing NOx is arranged in the exhaust section. 
         [0030]    The internal combustion engine is also assigned a control device  35  which is assigned sensors which sense various measurement variables and respectively determine the value of the measurement variables. The control device  35  is designed to determine manipulated variables as a function of at least one of the measurement variables, which manipulated variables can then be converted into one or more actuating signals for controlling actuator elements by means of corresponding actuator drives. The control device  35  can also be embodied as a device for testing an exhaust gas sensor SENS or comprise such a device. 
         [0031]    The actuator elements are, for example, the throttle valve  15 , the gas inlet valve  22  and gas outlet valve  24 , the injection valve  28  or the spark plug  30 . 
         [0032]    The sensors comprise, for example, a pedal position sensor  36  which senses a position of an accelerator pedal  38 . Furthermore, the internal combustion engine has an air mass flow rate sensor  40  which is arranged upstream of the throttle valve  15  and senses an air mass flow there. A temperature sensor  42  upstream of the throttle valve  15  senses an intake air temperature. An intake manifold pressure sensor  44  downstream of the throttle valve  15  is arranged in the collector  16  and senses an intake manifold pressure in the collector  16 . Furthermore, the internal compression engine comprises a crankshaft angle sensor  46  which senses a crankshaft angle to which a rotational speed of the internal combustion engine can be assigned. 
         [0033]    An exhaust gas probe  50  is arranged upstream of the exhaust gas catalytic converter  32 , which exhaust gas probe  50  senses a residual oxygen content of the exhaust gas and the measurement signal of said exhaust gas probe  50  is characteristic of the air/fuel ratio in the combustion chamber  26  of the cylinder Z 1  and upstream of the exhaust gas probe  50  before oxidation of the fuel. In addition, a lambda probe  52  is provided which is arranged downstream of the exhaust gas catalytic converter  32  and which senses a residual oxygen content of the exhaust gas and the measurement signal of said lambda probe  52  is characteristic of the air/fuel ratio in the combustion chamber  26  of the cylinder Z 1  and upstream of the lambda probe  52 . 
         [0034]    The exhaust gas probe  50  and the lambda probe  52  are preferably binary lambda probes. The exhaust gas probe  50  and/or the lambda probe  52  can, however, basically be embodied individually or else together as linear lambda probes. 
         [0035]    An exhaust gas probe  53  and the exhaust gas sensor SENS are arranged downstream of the exhaust gas catalytic converter  32 . The exhaust gas probe  53  senses a residual oxygen content of the exhaust gas and the measurement signal of said exhaust gas probe  53  is characteristic of the air/fuel ratio upstream of the exhaust gas probe  53 . The exhaust gas sensor SENS senses, in particular, an NOx concentration of the exhaust gas upstream of the exhaust gas probe  53 . In the text which follows, the exhaust gas sensor SENS and a diagnostic method are illustrated with respect to NOx by way of example. However, it is correspondingly also possible for one or more other components of the exhaust gas to be sensed by means of the exhaust gas sensor, and diagnostics can be performed on the exhaust gas sensor SENS with respect to this component or these components. 
         [0036]    The exhaust gas sensors SENS preferably comprises the exhaust gas probe  53 . This has the disadvantage that just a single sensor has to be made available for sensing the NOx concentration and the residual oxygen content of the exhaust gas. The exhaust gas sensor SENS is preferably designed to output a binary lambda signal. This is advantageous since the binary lambda signal is very sensitive to the residual oxygen content of the exhaust gas. However, the exhaust gas sensor SENS can basically comprise a linear lambda probe. 
         [0037]    Depending on the embodiment of the invention, any desired subset of the specified sensors may be present, or additional sensors may also be present. 
         [0038]    In addition to the cylinder Z 1 , further cylinders Z 2 , Z 3 , Z 4  are preferably also provided, corresponding actuator elements and, if appropriate sensors, being likewise assigned to said cylinders. 
         [0039]    The internal combustion engine is preferably embodied as a diesel internal combustion engine. However, the internal combustion engine can also be embodied in a different way, for example as a petrol internal combustion engine. 
         [0040]    The device for testing the exhaust gas sensor SENS is preferably arranged in the exhaust gas sensor SENS itself. A system comprising the exhaust gas sensor SENS and the device for testing the exhaust gas sensor SENS can be of such particularly compact design, and can, in particular be independent of the control device  35  of the internal combustion engine. 
         [0041]      FIG. 2  shows the exhaust gas sensor SENS in a cross section and a measuring circuit for operating the exhaust gas sensor SENS. The exhaust gas sensor SENS comprises a solid electrolyte E, which is preferably formed from zirconium dioxide ZrO 2 , and in which a heater H is arranged. An air duct with an air inlet LE for feeding in air from the surroundings is formed in the solid electrolyte E. In addition, a first chamber MK 1  and a second chamber MK 2  are formed in the solid electrolyte E. The first chamber MK 1  comprises a first main pump electrode HP 1 . The second chamber MK 2  comprises an auxiliary pump electrode M 1  and a measuring electrode M 2 . In addition, a second main pump electrode HP 2  is arranged on the outside of the solid electrolyte E, and a reference electrode REF is arranged in the air duct. The first chamber MK 1  and the second chamber MK 2  are separated from one another by a diffusion barrier DB 2 . A further diffusion barrier DB 1  forms a gas inlet for a gas whose NOx content is to be determined. The gas is formed, in particular, by exhaust gas of the internal combustion engine. The diffusion barrier DB 2  and the further diffusion barrier DB 1  are, in particular, permeable to nitrogen oxides NOx. In general, the diffusion barrier DB 2  and the further diffusion barrier DB 1  are also permeable to oxygen and/or other components of the gas, and are, in particular, permeable to the exhaust gas of the internal combustion engine. 
         [0042]    The measuring circuit comprises a first regulator C 1 , a first voltage-controlled power source UI 1  and preferably a first conditioning device K 1 . The auxiliary pump electrode M 1  and the reference electrode REF of the exhaust gas sensor SENS are coupled to an input of the first regulator C 1  via the first conditioning device K 1  for the purpose of sensing a first voltage V 1  between the auxiliary pump electrode M 1  and the reference electrode REF. On the output side, the first regulator C 1  is coupled via the first voltage-controlled power source UI 1  to the second main pump electrode HP 2  and to the auxiliary pump electrode M 1  for the purpose of driving a first pump current IP 1  between the second main pump electrode HP 2  and the auxiliary pump electrode M 1 . The first regulator C 1  is, in particular, designed to regulate the first pump current IP 1  as a function of the first voltage V 1 . The first regulator C 1  is also preferably embodied in such a way that during the testing of the exhaust gas sensor SENS the first pump current IP 1  is set or adjusted to a predefined pump current which is, in particular, predefined as a constant. 
         [0043]    The measuring circuit also comprises a second regulator C 2 , a second voltage-controlled power source UI 2  and preferably a second conditioning device K 2 . The measuring electrode M 2  and the reference electrode REF of the exhaust gas sensor SENS are coupled to an input of the second regulator C 2  via the second conditioning device K 2  for the purpose of sensing a second voltage V 2  between the measuring electrode M 2  and the reference electrode REF. On the output side, the second regulator C 2  is coupled via the second voltage-controlled power source UI 2  to the second main pump electrode HP 2  and to the measuring electrode M 2  for the purpose of driving a second pump current which forms a measuring current Im, between the second main pump electrode HP 2  and the measuring electrode M 2 . The second regulator C 2  is, in particular, designed to regulate the second pump current, that is to say the measuring current Im, as a function of the second voltage V 2 . The second regulator C 2  is, in particular, designed to regulate the measuring current Im in such a way that the second voltage V 2  corresponds to a predefined voltage. The predefined voltage is, in particular, predefined in such a way that a stoichiometric gas mixture is formed in the surroundings of the measuring electrode M 2 , that is to say the surroundings of the measuring electrode M 2  are essentially free of oxygen. The predefined voltage can be, for example, approximately between 400 and 450 millivolts. However, the predefined voltage can also be higher than 450 millivolts or lower than 400 millivolts. 
         [0044]    In addition, the measuring circuit comprises a further regulator C 0 , a further voltage controlled power source UI 0  and preferably a further conditioning device K 0 . The first main pump electrode HP 1  and the reference electrode REF of the exhaust gas sensor SENS are coupled via the further conditioning device K 0  to an input of the further regulator C 0  for the purpose of sensing a further voltage V 0  between the first main pump electrode HP 1  and the reference electrode REF. On the output side, the further regulator C 0  is coupled via the further voltage-controlled power source UI 0  to the first and second main pump electrodes HP 1 , HP 2  for the purpose of driving a further pump current IP 0  between the first and second main pump electrodes HP 1 , HP 2 . The further regulator C 0  is, in particular, designed to regulate the further pump current IPO as a function of the further voltage V 0 . The first chamber MK 1  forms, together with the first and second main pump electrodes HP 1 , HP 2  and the reference electrode REF, in particular a binary lambda probe and can represent, in particular, the exhaust gas probe  53 . 
         [0045]    The pump currents, that is to say the first, second and further pump currents IP 1 , IP 0 , cause oxygen to be transported through the solid electrolyte E and into the respective chamber or out of the respective chamber, said solid electrolyte E being heated to a suitable temperature for this by the heater H. The voltages, that is to say the first, second and further voltages V 1 , V 2 , V 0  are, in particular, Nernst voltages and are dependent on the respective oxygen concentrations in the first chamber MK 1 , the second chamber MK 2  and the surrounding air at the reference electrode REF. In order to test the exhaust gas sensor SENS, the first and further pump currents IP 1 , IP 0  are preferably set or adjusted in such a way that a predefined oxygen concentration, which is in particular, greater than zero parts per million, occurs in the second chamber MK 2 . The oxygen which is located in the second chamber MK 2  acts on the first voltage V 1 , and, in the case of a sufficiently functionally capable exhaust gas sensor SENS, said oxygen acts on the second pump current, that is to say the measuring current Im. 
         [0046]    For example, the measuring electrode M 2  is provided with a protective layer which is permeable to nitrogen oxides NOx and oxygen. However, contamination of the measuring electrode M 2  and, in particular, of the protective layer may adversely affect the sensitivity of the measuring electrode M 2  to oxygen or the permeability of he protective layer to oxygen. The contamination of the exhaust gas sensor SENS causes the measuring current Im to generally deviate from the measuring current Im which is expected for the fault-free exhaust gas sensor SENS. The measuring current Im of the contaminated exhaust gas sensor SENS is, in particular, too low. 
         [0047]    However, the protective layer may, for example, become detached from the measuring electrode M 2 . As a result, it is generally possible for more oxygen to arrive at the measuring electrode M 2  than when the protective layer is intact. As a result of the detachment of the protective layer from the measuring electrode M 2 , the measuring current Im generally deviates from the measuring current Im which is expected for the fault-free exhaust gas sensor SENS. The measuring current Im is in particular too large when the protective layer is detached. 
         [0048]    In order to carry out the diagnostics, that is to say the testing of the exhaust gas sensor SENS, a program can be stored in a program memory of the control device  35  and run while the internal combustion engine is operating. FIG.  4  shows a flow chart of a method for testing the exhaust gas sensor SENS and the program.  FIG. 3  shows a diagram with diagnostic values DIAG. 
         [0049]    The method starts at a step S 1 . A step S 2  can be provided for testing whether at least one predefined diagnostic condition B applies. The at least one predefined diagnostic condition B can comprise, in particular, a predefined operating state of the internal combustion engine, for example overrun conditions, or generally an operating state with an essentially nonvariable load and as a result an essentially nonvariable and predefined oxygen concentration in the exhaust gas. The at least one predefined diagnostic condition can also comprise, in particular, a small NOx measured value of the exhaust gas sensor SENS which is below a predefined NOx threshold value. The predefined NOx threshold value can, for example, be approximately  50  parts per million. However, the predefined NOx threshold value can likewise be higher or lower than 50 parts per million. It is also possible for further or other diagnostic conditions B to be provided. Preferably, the predefined operational state lasts for the duration of the diagnostics performed on the exhaust gas sensor SENS. The duration is, for example, approximately five seconds but can also be longer or shorter. 
         [0050]    A step S 3  can be provided in which the further pump current IPO is sensed. The further pump current IPO is dependent on the oxygen concentration in the first chamber MK 1  and is preferably representative of the oxygen concentration in the first chamber MK 1 . The further pump current IPO is preferably sensed at the start of the diagnostics. The further pump current IP 0  can, in particular, also represent an oxygen concentration of the exhaust gas. It is possible to provide for this purpose that the further voltage V 0  be adjusted to a predefined value. The step S 3  can, in particular, be dispensed with if the testing of the exhaust gas sensor SENS is carried out in an operating state of the internal combustion engine for which the oxygen concentration in the exhaust gas is known. This can, for example, be the case during overrun conditions. 
         [0051]    In a step S 4 , the first pump current IP 1  and the further pump current IP 0  are predefined. The first pump current IP 1  is preferably predefined reduced in absolute value compared to a normal sensor operating mode of the exhaust gas sensor SENS outside the diagnostics. In a step S 5 , the first voltage V 1  which is set is sensed. The first voltage V 1  is dependent on the oxygen concentration in the second chamber MK 2  and is preferably representative of the oxygen concentration in the second chamber MK 2 . The associated oxygen concentration in the second chamber MK 2  is preferably assigned to the sensed first voltage V 1 . 
         [0052]    In general, the oxygen concentration, which can be determined as a function of the first voltage V 1 , is only a rough approximation compared to the oxygen concentration which can in principle be determined as a function of the measuring current Im. However, said oxygen concentration is insensitive to contamination and detachment of the protective layer of the measuring electrode M 2 . 
         [0053]    In a step S 6 , a set point diagnostic value SDIAG is determined as a function of the first voltage V 1 . If the further pump current IPO has been sensed in the step S 3 , in the step S 6  the set point diagnostic value SDIAG is preferably determined as a function of the first voltage V 1  and the further pump current IPO which was sensed in step S 3 . For example, in a step S 6   a  a predefined factor F and/or a predefined shift value O are selected as a function of the further pump current IPO. The respectively associated predefined factor F and/or predefined shift value O, from which a selection can be made as a function of the further pump current IP 0  sensed in the step S 3 , are preferably predefined and stored for various values or value ranges of the further pump current IP 0 . If the further pump current IPO has not been sensed, the predefined factor F and the predefined shift value are preferably permanently predefined independently of the further pump current IP 0 , being predefined in particular as constants. In a step S 6   b,  the set point diagnostic value SDIAG is determined, for example, as a function of the predefined factor F, the first voltage V 1  and the predefined shift value O, for example as a function of multiplication of the predefined factor F by the first voltage V 1  and addition of the predefined shift value O: SDIAG=f(F*V 1 +O), and in particular: SDIAG=F*V 1 +O. 
         [0054]    In  FIG. 3 , the set point diagnostic value SDIAG is represented as a function of the first voltage V 1  as a characteristic curve which is, in particular, a straight line. The predefined factor F and the predefined shift value O are preferably determined experimentally and in particular by calibration.  FIG. 3  illustrates, by way of example, three calibration values KAL, which have been sensed, for example, during the calibration of the exhaust gas sensor SENS. The calibration is preferably carried out in a new state of the exhaust gas sensor SENS and is preferably carried out within the scope of the manufacture of the exhaust gas sensor SENS. The characteristic curve results, for example, as a regression straight line as a function of the calibration values KAL, on which straight line the predefined factor F can be determined easily as a function of the gradient thereof, and the predefined shift value O can easily be determined. The calibration values KAL which each form the basis for such a characteristic curve are preferably sensed for a predefined oxygen concentration in a gas which is used for the calibration, if appropriate instead of the exhaust gas of the internal combustion engine. By varying the predefined oxygen concentration in the gas it is possible for various characteristic curves with different predefined factors F and/or predefined shift values O to be formed between which, according to step S 6   a,  it is possible to select as a function of the further pump current IPO. In this way a high degree of accuracy of the set point diagnostic value SDIAG is possible. 
         [0055]    In a step S 7 , the measuring current Im is sensed. In a step S 8 , an actual diagnostic value IDIAG which is associated with the measuring current Im is determined. An associated oxygen concentration at the measuring electrode M 2  in the second chamber MK 2  which, however, under certain circumstances, does not correspond to the actual oxygen concentration owing, for example, to contamination or detachment of the protective layer, is preferably assigned to the measuring current Im or the diagnostic value IDIAG. In a step S 9 , it is tested whether deviation of the oxygen concentration assigned to the actual diagnostic value IDIAG from the oxygen concentration assigned to the set point diagnostic value SDIAG occurs, by testing, for example, whether a deviation of the actual diagnostic value IDIAG from the set point diagnostic value SDIAG exceeds a predefined threshold value. If this is not the case, the exhaust gas sensor SENS is detected in a step S 10  as free of faults and the testing of the exhaust gas sensor SENS is ended in a step S 11 . 
         [0056]    However, if it is detected in the step S 9 , for example, that the actual diagnostic value, IDIAG is lower than the set point diagnostic value SDIAG by more than a predefined lower threshold value USW, that is to say that the oxygen concentration which is assigned to the actual diagnostic value IDIAG is too low compared to the oxygen concentration assigned to the set point diagnostic value SDIAG, in a step S 12  a first fault ERR 1  is detected as a fault ERR of the exhaust gas sensor SENS. The first fault ERR 1  corresponds, in particular, to the contamination of the exhaust gas sensors SENS. If appropriate, a corresponding entry is made in a fault memory and/or the fault ERR is signaled. The testing of the exhaust gas sensor SENS is ended in the step S 11 . 
         [0057]    If it is detected in the step S 9  that, for example, the actual diagnostic value IDIAG is higher than the set point diagnostic value SDIAG by more than a predefined upper threshold value OSW, that is to say that the oxygen concentration assigned to the actual diagnostic value IDIAG is too high compared to the oxygen concentration assigned to the set point diagnostic value SDIAG, in a step S 13  a second fault ERR 2  is detected as a fault ERR of the exhaust gas sensor SENS. The first fault ERR 2  corresponds, in particular, to the detachment of the protective layer from the measuring electrode M 2  of the exhaust gas sensor SENS. If appropriate, a corresponding entry is made in the fault memory and/or the fault ERR is signaled. The testing of the exhaust gas sensor SENS is ended in the step S 11 . 
         [0058]    The relationship between diagnostic values DIAG and the assigned oxygen concentration can alternatively also be predefined inversely, that is to say, for example, the first fault ERR 1  is detected if it is detected in the step S 9  that the actual diagnostic value IDIAG is higher than the set point diagnostic value SDIAG by more than the predefined lower threshold value USW, which means that the oxygen concentration assigned to the actual diagnostic value IDIAG is too low compared to the oxygen concentration assigned to the set point diagnostic value SDIAG, and, for example, the second fault ERR 2  is detected if it is detected in the step S 9  that the actual diagnostic value IDIAG is lower than the set point diagnostic value SDIAG by more than the predefined upper threshold value OSW, which means that the oxygen concentration assigned to the actual diagnostic value IDIAG is too high compared to the oxygen concentration assigned to the set point diagnostic value SDIAG.