Abstract:
A method is described for diagnosing a heatable exhaust gas sensor of an internal combustion engine, in which a predefined chronologically varying or constant voltage or a predefined chronologically varying or constant current is generated with the aid of a voltage source, the voltage or the current is applied to terminals of the exhaust gas sensor, a current or applied voltage, which flows through the voltage source when the voltage or the current is applied, is detected, and the current or the voltage is analyzed to diagnose the exhaust gas sensor. To diagnose the exhaust gas sensor, which permits a reliable and accurate diagnosis of the exhaust gas sensor and allows a statement about the type of a possibly existing error on the exhaust gas sensor, the method is executed independently of a control and/or regulating unit of the internal combustion engine, an operating temperature of the exhaust gas sensor being regulated to a predefined temperature value with the aid of a regulating element, which is separate from the control and/or regulating unit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for diagnosing a heatable exhaust gas sensor of an internal combustion engine and a diagnostic device. 
       BACKGROUND INFORMATION 
       [0002]    Equipping internal combustion engines, in particular internal combustion engines for motor vehicles, with one or more exhaust gas sensors is generally known. The exhaust gas sensors are typically connected to a control and/or regulating unit of the internal combustion engine, so that the control and/or regulating unit may detect information about the composition of exhaust gases exiting from combustion chambers of the internal combustion engine. Typically at least one lambda sensor, which may be used to detect an oxygen concentration in the exhaust gas, is provided as an exhaust gas sensor in an internal combustion engine. This allows a conclusion to be drawn about an air/fuel ratio in the combustion chamber, so that the internal combustion engine may be regulated in such a way, for example, that emission regulations are met. 
         [0003]    The lambda sensors may be divided into so-called bistable sensors and broadband sensors. The broadband sensors may in turn be designed to be single-cell broadband sensors or to be dual-cell broadband sensors. A bistable sensor has a rather high sensitivity for an excess-air ratio of the exhaust gas which is in the range of λ=1. For excess-air ratios which are not in the range of approximately λ=1, the sensitivity of the bistable sensors is relatively low. Therefore, in the case of a continually rising excess-air ratio in the exhaust gas, a jump of an output signal generated by a bistable sensor results as soon as the excess-air ratio enters the range of approximately λ=1. In contrast, broadband lambda sensors have a relatively high sensitivity also outside the range of the excess-air ratio around the value λ=1. 
         [0004]    Modern internal combustion engines typically have one or two lambda sensors, bistable and/or broadband sensors being used for gasoline engines. Diesel engines predominantly have broadband lambda sensors. 
         [0005]    During operation of the internal combustion engine, the control and/or regulating unit detects sensor signals which are generated by the lambda sensor or the lambda sensors and additional sensors of the internal combustion engine and operates the internal combustion engine as a function of these sensor signals. In order to be able to recognize errors in the sensors, the control and/or regulating unit checks the individual sensor signals during the operation of the internal combustion engine. In this case, the signals are typically checked as to whether electrical errors (e.g., short-circuits or line interruptions) exist. For this purpose, for example, it may be checked whether the sensor signals are in permissible value ranges. In addition, the control and/or regulating unit typically checks whether there are system errors. A system error is recognized, for example, if the variables detected with the aid of different sensors contradict one another. If the control and/or regulating unit recognizes an electrical error and/or a system error, it registers the occurrence of the error in an error memory. 
         [0006]    Known diagnostic methods, which are performed, for example, to prepare for repairs or during maintenance work on the internal combustion engine or on a motor vehicle in which the internal combustion engine is installed, resort to the information stored in the error memory, for example. In this way, conclusions about the functionality of a lambda sensor may be drawn to a certain extent. Since there are complex interactions between various components of the internal combustion engine during operation of the internal combustion engine, however, a sufficiently secure and reliable diagnosis of an individual lambda sensor is not possible. For example, if an electrical error is recognized, it typically cannot be reliably established whether the error is based on a defect of the lambda sensor or whether the control and/or regulating unit, in particular an analysis circuit for the sensor signals of the lambda sensor, is defective. In addition, in many cases system errors may not be definitely assigned to a specific sensor, for example, a specific lambda sensor. The risk exists that the lambda sensor will be incorrectly recognized as defective, although in actuality another component of the internal combustion engine is not functioning correctly, in particular another sensor of the internal combustion engine. If a defect occurs in the internal combustion engine, cumbersome error searches may therefore occur if known diagnostic methods are used, until the actually defective component is finally identified. Reliable conclusions about the type of the error of the lambda sensor are practically impossible to be drawn in the case of known diagnostic methods. 
         [0007]    Single-cell and dual-cell broadband lambda sensors are discussed, for example, in DE 10 2006 014 266 A1. Furthermore, detecting a leakage current between an electrode of the lambda sensor and a heating element of the lambda sensor is discussed in DE 197 16 173 A1. 
       SUMMARY OF THE INVENTION 
       [0008]    The exemplary embodiments and/or exemplary methods of the present invention are based on the object of providing a method for diagnosing an exhaust gas sensor of an internal combustion engine, which permits reliable and accurate diagnosis of the exhaust gas sensor and allows a statement about the type of a possibly existing error in the exhaust gas sensor. With respect to its device aspects, the object is to provide a diagnostic device which is configured for performing the method. 
         [0009]    These objects are each achieved by the features of the independent claims. The exhaust gas sensor may be a lambda sensor, in particular a bistable sensor, a single-cell broadband sensor, or a dual-cell broadband sensor. In the case of the diagnosis of the exhaust gas sensor with the aid of the method according to the present invention, various parameters of the exhaust gas sensor may be checked largely independently of other components of the internal combustion engine. In particular, interactions with other sensors of the internal combustion engine are prevented. Access to an error memory of the control and/or regulating unit is not necessary. The exhaust gas sensor is brought to a defined operating point by the regulation of the temperature of the exhaust gas sensor, in particular the temperature of a sensor element of the exhaust gas sensor, so that the diagnosis delivers results having a high validity and reproducibility. 
         [0010]    The method may be executed when the internal combustion engine is shut down and is not in operation. In this case, the exhaust gas sensor may remain installed in the internal combustion engine. However, the diagnosis as per the method according to the exemplary embodiments and/or exemplary methods of the present invention may also be performed on an exhaust gas sensor which has been removed from the internal combustion engine. 
         [0011]    Overall, a rapid and reliable check of the exhaust gas sensor for errors is made possible by the method according to the present invention. In addition, a relatively detailed appraisal of the exhaust gas sensor may be performed, which may be carried out not only within the scope of repair or maintenance work, but rather also at the end of a manufacturing process of the exhaust gas sensor, the internal combustion engine, or a motor vehicle in which the internal combustion engine is installed. It is also conceivable that the method is performed when it has been shown that a motor vehicle which was just manufactured is not functioning correctly, so that the type of the error may be analyzed more precisely with the aid of the method according to the present invention. Such a more precise analysis is also referred to as a “zero kilometer appraisal.” Furthermore, lambda sensors of motor vehicles which the customer has complained about while claiming a manufacturer&#39;s warranty may be checked with the aid of the method according to the present invention. 
         [0012]    The voltage may be applied to the terminals which are connected to electrodes of a cell, in particular a pump cell (in the case of a dual-cell sensor) or a combined pump and measuring cell (in the case of a single-cell sensor) of the exhaust gas sensor, so that a pump current which flows through the cell in the case of an intact exhaust gas sensor is detected as the current. By analyzing the detected pump current, it may be checked whether the exhaust gas sensor is functional or whether the exhaust gas sensor has an error. 
         [0013]    The voltage may be varied step-by-step in alternating directions in such a way that the voltage successively has various voltage values, and associated current values of the current are detected for at least two of these voltage values. The two voltage values, for which the associated current values are detected, may be equal. 
         [0014]    It may be in this case that a hysteresis with respect to the dependence between the applied voltage and the detected current is checked by analyzing the current values, which may be by comparing the current values with one another. If precisely two current values are detected for two equal voltage values, which are applied at different points in time, a difference between the two current values may be used as a measure for the hysteresis. If the absolute value of the difference is greater than a predefined threshold value, a defect, in particular blackening, i.e., ceramic reduction as a result of overloads/excessive voltages, on one of the electrodes of the cell may be inferred. 
         [0015]    Furthermore, the voltage may be applied to the terminals which are connected to a trim resistor of the exhaust gas sensor. In this way, on the one hand, it may be checked whether the trim resistor is correctly connected within the exhaust gas sensor to the terminals of the exhaust gas sensor to which the voltage is applied or via a connection cable of the exhaust gas sensor. If the current is outside a permissible range, a poor contact of the trim resistor or an interruption of the connection between one of the terminals and the trim resistor or a shunt parallel to the trim resistor is inferred. In general, a shunt is understood as an undesirable electrically conductive path which runs parallel to a desired electrically conductive main path. If the current is within the permissible range, the value of the trim resistor may be ascertained on the basis of the current. 
         [0016]    In this case, as a function of a value of the trim resistor that a setpoint value for the pump current in air, for example, may be ascertained, a positive pump voltage may be applied as the voltage, and the exhaust gas sensor may be checked as a function of the setpoint value and the pump current. A quotient between the pump current and the setpoint value may be ascertained in this case. If the absolute value of the quotient is greater than a predefined setpoint value, for example, a crack in a diffusion barrier or in the sensor ceramic of the cell or an electrical shunt between the electrodes of the cell is recognized. If the absolute value of the quotient is less than a further setpoint value, contaminants of the diffusion barrier (“sooting”) may be recognized. 
         [0017]    As a further check, it may be provided that a negative pump voltage is applied as the voltage, an inverted pump current is detected as the current, and it is checked whether the current is in a predefined permissible range. An excessively small current indicates a contamination of a protective layer of the exhaust gas sensor or insufficient heating of the exhaust gas sensor. If the current is excessively large, it is possible there is an electrical shunt between the electrodes or damage to or a lack of the protective layer of the sensor. 
         [0018]    In order to recognize a shunt due to contamination, specifically sooting in an area between a sensor element and a housing of the exhaust gas sensor, the voltage may be applied between an electrode of the cell, which may be an inner electrode of a pump cell, and an electrically conductive housing part of the exhaust gas sensor, a housing current may be detected as the current, and it may be checked whether the housing current is less than or equal to a predefined maximum value. If the current exceeds the maximum value, the method establishes that soot or other, in particular metallic, deposits have accumulated between the sensor element and the housing, in particular a protective tube of the housing. 
         [0019]    The above-described checks of the exhaust gas sensor often include a comparison of the detected current or a variable, which is formed as a function of the detected current, with predefined threshold values or predefined permissible ranges. Since different types of exhaust gas sensors are used, the threshold values or the permissible ranges must be predefined as a function of the type of the exhaust gas sensor. Furthermore, the regulation of the operating temperature must often be adapted to the type of the exhaust gas sensor. For this purpose, the type of the exhaust gas sensor may be ascertained as a function of manual inputs by a user. However, at least one measured variable which characterizes a cell resistance of the at least one cell of the exhaust gas sensor, which may be the cell resistance of a measuring cell of the exhaust gas sensor, may be detected or ascertained and a type of the exhaust gas sensor may be ascertained as a function of the measured variable. It was recognized that the individual types of the exhaust gas sensor differ in particular in their cell resistance, so that an assignment of the type to the cell resistance is possible. Operating errors by the user are largely prevented by the automatic ascertainment of the type of the exhaust gas sensor. 
         [0020]    To ascertain the cell resistance, it may be provided that at least one measuring voltage is applied to the cell as the voltage and the current through the cell is detected as the measured variable for each measured voltage. In this way, the type of the exhaust gas sensor may be ascertained more reliably, since the resistance is not only ascertained for one voltage, but rather for multiple voltages. 
         [0021]    In this case that at least two measuring voltages of different polarity may be applied to the cell in chronological succession. For example, in this way a statement about the ohmic resistance of the cell ceramic may be derived in the first case and a statement about the diffusion resistance of the oxygen transport to the electrode may be derived in the second case. In this way, age-related changes of the exhaust gas sensor of the same type may be differentiated from differences between exhaust gas sensors of various types. In this way, errors in the automatic recognition of the type of the exhaust gas sensor due to aging or wear of the exhaust gas sensor are at least largely prevented. 
         [0022]    In a specific embodiment of the present invention, the or each measured variable is compared with a threshold value and the type of the exhaust gas sensor is ascertained as a function of this comparison. This means that a result of the comparison is ascertained for each measured variable and the comparison results are logically linked to one another to ascertain the type of the exhaust gas sensor. 
         [0023]    The oxygen content of the gas to which the exhaust gas sensor is subjected forms a further influencing factor on the cell resistance. An exhaust pipe of a typical internal combustion engine, in which the exhaust gas sensor is installed, is typically sealed well in relation to the ambient air in such a way that after the shutdown of the internal combustion engine, oxygen-poor exhaust gas remains in the exhaust pipe and a gas exchange with the surroundings occurs relatively slowly. Therefore, the exhaust gas sensor may be subject to an oxygen-poor gas (excess-air ratio λ&lt;0) during execution of the method. In order to eliminate an interfering influence as extensively as possible during the recognition of the type of the exhaust gas sensor, prior to the application of the at least one measuring voltage to the cell, a cell voltage generated by the exhaust gas on the cell may be detected and at least one measuring voltage is predefined as a function of the cell voltage. It may be provided in this case that the at least one measuring voltage is increased by the detected cell voltage. 
         [0024]    A diagnostic device having the features of Claim  14  is proposed as a further approach to the above-mentioned object. With the aid of such a diagnostic device, the exhaust gas sensor may be checked particularly simply. For this purpose, when the engine is shut down, an electrical connection between the exhaust gas sensor and the control and/or regulating unit of the internal combustion engine is disconnected and the terminals of the exhaust gas sensor are connected to the diagnostic device. In this way, an isolated diagnosis of the exhaust gas sensor is made possible. An error on the exhaust gas sensor may either be definitively established or reliably precluded in this way. 
         [0025]    The diagnostic device may be configured to execute the method according to the present invention and may therefore implement all advantages of the method according to the present invention. In particular, the diagnostic device may have a programmable computer, which is programmed to execute the method according to the present invention. 
         [0026]    Further features and advantages of the exemplary embodiments and/or exemplary methods of the present invention arise from the following description, in which exemplary specific embodiments of the present invention are explained in greater detail on the basis of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  shows a diagnostic device, which is connected to a dual-chamber broadband lambda sensor, in a schematic view. 
           [0028]      FIG. 2  shows a view similar to  FIG. 1 , the lambda sensor being a single-chamber broadband lambda sensor. 
           [0029]      FIG. 3  shows a part of a flow chart as an exemplary embodiment of a method for diagnosing the lambda sensor shown in  FIGS. 1 and 2 . 
           [0030]      FIG. 3  shows another part of a flow chart as an exemplary embodiment of a method for diagnosing the lambda sensor shown in  FIGS. 1 and 2 . 
           [0031]      FIG. 4  shows another part of a flow chart as an exemplary embodiment of a method for diagnosing the lambda sensor shown in  FIGS. 1 and 2 . 
           [0032]      FIG. 5  shows another part of a flow chart as an exemplary embodiment of a method for diagnosing the lambda sensor shown in  FIGS. 1 and 2 . 
           [0033]      FIG. 6  shows another part of a flow chart as an exemplary embodiment of a method for diagnosing the lambda sensor shown in  FIGS. 1 and 2 . 
           [0034]      FIG. 7  shows another part of a flow chart as an exemplary embodiment of a method for diagnosing the lambda sensor shown in  FIGS. 1 and 2 . 
           [0035]      FIG. 8  shows a more detailed view of a step of the method from  FIGS. 3 through 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    The schematic view of  FIG. 1  shows a dual-cell broadband lambda sensor  11 , which is connected to a diagnostic device  15  via an electrical connection in the form of a plug connector  13 . Lambda sensor  11  is part of an exhaust gas system of an internal combustion engine (not shown). It may be situated upstream or downstream from an exhaust gas catalytic converter in an exhaust pipe of the exhaust gas system in the flow direction, for example. Lambda sensor  11  may also be temporarily removed from the internal combustion engine for the purpose of a diagnosis, however. It is also conceivable that lambda sensor  11  is provided for the initial installation in the internal combustion engine and is connected to diagnostic device  15  for an initial function test. The initial function test may also be carried out when lambda sensor  11  is already installed. 
         [0037]    Lambda sensor  11  has a pump cell  17 . Pump cell  17  includes an outer pump electrode  19 , which is connected to a terminal of plug connector  13  identified by “APE.” An inner pump electrode  21  of pump cell  17  is connected to a terminal IPN of plug connector  13 . A first solid-state electrolyte  23 , which is made of zirconium dioxide, is located between outer pump electrode  19  and inner pump electrode  21 . If lambda sensor  11  is installed in the exhaust gas system, a side of pump cell  17  which is delimited by outer pump electrode  19  faces toward an inner chamber of the exhaust pipe of the internal combustion engine, whereas a side of pump cell  17  which is delimited by the inner pump electrode faces toward a diffusion gap (not shown) provided in the interior of lambda sensor  11 . Pump cell  17  is therefore located between a side of lambda sensor  11  which faces toward the inner chamber of the exhaust pipe and the diffusion gap of lambda sensor  11 . 
         [0038]    A measuring cell, which is typically referred to as a Nernst cell  25 , is situated between the diffusion gap and a reference air duct (not shown) of lambda sensor  11 , which is typically connected to ambient air. Nernst cell  25  has a second solid-state electrolyte  27 , on whose side facing toward the diffusion gap a Nernst electrode  29  is situated, which is electrically connected to terminal IPN of plug connector  13 . A reference electrode  31  of Nernst cell  25  is situated on a side of second solid-state electrolyte  27  facing toward the reference air duct. Reference electrode  31  is electrically connected to a terminal RE of plug connector  13 . In addition, lambda sensor  11  has a heating element  33 , which is connected to two terminals H+ and H− of plug connector  13 . Heating element  33  and both cells  17  and  25  are integrated into a sensor element of lambda sensor  11 , so that heating element  33  is thermally coupled to cells  17 ,  25 , in particular to their solid-state electrolytes  23 ,  27 . 
         [0039]    Lambda sensor  11  is constructed according to a suitable manufacturing technology. For example, lambda sensor  11  may be designed as a so-called finger sensor or may be manufactured using a planar technology. Independently of the employed manufacturing technology, lambda sensor  11  has a housing  35 , which has an electrically conductive housing part  37 , which may be made of metal, for example. Electrically conductive housing part  37  is connected to diagnostic device  15 . 
         [0040]    Furthermore, a trim resistor  39  is situated in lambda sensor  11 , a first terminal of trim resistor  39  being connected to terminal APE of plug connector  13  and a second terminal of trim resistor  39  being connected to terminal RT of plug connector  13 . Trim resistor  39  may have a value of approximately 30 ohm to 300 ohm, for example. The value of trim resistor  39  is typically established immediately after the manufacture of the lambda sensor. For this purpose, trim resistor  39  is connected in parallel to a measuring resistor in the regulating electronics. The trim resistor is then set in such a way that a predefined current (e.g., 2.54 mA) results through the measuring resistor when lambda sensor  11  is subjected to a gas having the excess-air ratio λ=1. During operation of lambda sensor  11 , manufacturing tolerances of lambda sensor  11  may therefore be at least largely compensated for with the aid of trim resistor  39 . 
         [0041]    Diagnostic device  15  has a first voltage source  41 , which is controllable by a control unit  43  of diagnostic device  15 . First voltage source  41  is connected in series to a first current sensor  45 . First current sensor  45  is connected to control unit  43 , so that control unit  43  may detect a current I 1  flowing through first voltage source  41 . A terminal of first current sensor  45  which faces away from voltage source  41  is connected to terminal APE of plug connector  13 . A side of first voltage source  41  which faces away from first current sensor  45  is connected to a terminal of a first switch element  47  and a second switch element  49 . A further terminal of first switch element  47  is connected to terminal RT of plug connector  13 . A further terminal of second switch element  49  is connected to terminal IPN of plug connector  13 . 
         [0042]    A voltage sensor  52  is situated between terminals APE and RE, which is connected to control unit  43  in such a way that it may detect a voltage U M  applied between terminals APE and RE. 
         [0043]    Furthermore, diagnostic device  15  has a second voltage source  51 , which is connected in series to a second current sensor  53 . Second voltage source  51  is controllable and is connected to control unit  43  in such a way that it may set a voltage U 2  generated by second voltage source  51  during its operation. Second current sensor  53  is coupled to control unit  43  in such a way that control unit  43  may detect a current I 2  flowing through second voltage source  51 . A terminal of second current sensor  53  which faces away from second voltage source  51  is connected to terminal IPN of plug connector  13 . A terminal of second voltage source  51  which faces away from second current sensor  53  is connected to a third switch element  55  and a fourth switch element  57 . A terminal of third switch element  55 , which is not directly connected to second voltage source  51 , is connected to housing part  37  of lambda sensor  11 , and a terminal of fourth switch element  57 , which is not directly connected to second voltage source  51 , is connected to terminal RE of plug connector  13 . Each switch element  47 ,  49 ,  55 ,  57  is coupled to control unit  43 , so that control unit  43  may individually activate individual switch elements  47 ,  49 ,  55 ,  57  (corresponding connections are not shown in  FIG. 1  for the sake of clarity). As a whole, switch elements  47 ,  49 ,  55 ,  57  form a switch configuration for connecting voltage sources  41 ,  51  and associated current sensors  45 ,  53  to individual terminals APE, RT, IPN, RE of plug connector  13  and to the housing. In other specific embodiments of diagnostic device  15 , the switch configuration is constructed in another way. The switch elements may be situated on other terminals of plug connector  13 , for example, also between a signal electrode and a heater electrode to check the internal leakage current. A different number of switch elements may also be provided. In addition, it is conceivable to provide only one or more than two voltage sources instead of two voltage sources  41 ,  51  and to increase or decrease the number of the switch elements accordingly. Switch elements  47 ,  49 ,  55 ,  57  may be implemented in any desired way (e.g., semiconductor switches or switch relays). 
         [0044]    Furthermore, diagnostic device  15  has a regulating element  59  for regulating a temperature of lambda sensor  11  on the basis of an internal resistance of Nernst cell  25 . Regulating element  59  is connected to both terminals H+ and H− of plug connector  13 , which are connected to heating element  33  of lambda sensor  11 . Regulating element  59  is connected to control unit  43 , so that control unit  43  may control regulating element  59  to predefine a setpoint value, for example. 
         [0045]    In the view of  FIG. 2 , the exhaust gas sensor is designed as a single-cell broadband lambda sensor  61 . Instead of pump cell  17  and Nernst cell  25 , single-cell broadband lambda sensor  61  has a combined pump and Nernst cell  63 . Therefore, only first solid-state electrolyte  23  is provided in this sensor  61 . An outer electrode  65  is situated on a side of first solid-state electrolyte  23  which faces toward the inner chamber of the exhaust pipe when sensor  61  is installed. An inner electrode  67  is situated on a side of first solid-state electrolyte  23  which faces away from the inner chamber. Outer electrode  65  is electrically connected to a terminal ALE of plug connector  13 , and inner electrode  67  is electrically connected to a terminal IPE of plug connector  13 . 
         [0046]    Apart from the fact that only one cell  63  is provided in single-cell broadband lambda sensor  61 , it has the same design in principle as dual-cell broadband lambda sensor  11  shown in  FIG. 1 . The respective parts of single-cell broadband lambda sensor  61  are therefore provided with the same reference numerals and will not be explained in detail once again. Simplified diagnostic device  15  shown in  FIG. 2  may be connected to single-cell broadband lambda sensor  61 . In diagnostic device  15  shown in  FIG. 2 , third switch element  55  and fourth switch element  57 , which are shown in  FIG. 1 , are not provided. It is also possible to use diagnostic device  15  shown in  FIG. 1  in connection with single-cell broadband lambda sensor  61 . Terminal RT of diagnostic device  15  may remain free in this case, and combined pump and Nernst cell  63  is connected using its terminal ALE to terminal APE of diagnostic device  15  and using its terminal IPE to terminals IPN and RE of diagnostic device  15 . 
         [0047]    In a specific embodiment (not shown), single-cell broadband lambda sensor  61  also has trim resistor  39 . It may be situated, for example, between terminal ALE and terminal RT, which is not provided in lambda sensor  61  shown in  FIG. 2 . 
         [0048]    A method  71  for diagnosing an exhaust gas sensor, in particular dual-cell lambda sensor  11  or single-cell lambda sensor  61 , is explained in greater detail hereafter on the basis of the flow chart shown in  FIGS. 3 through 7 . This method  71  may be performed using the diagnostic device shown in  FIGS. 1 and 2 , controlled by its control unit  43 . As a deviation, method  71  may also be performed in another way, in particular using differently constructed diagnostic devices and/or other, e.g., dynamic voltage-time programs or current-time programs, which are sinusoidal, for example. 
         [0049]    When diagnostic device  15  is used, lambda sensor  11  must be electrically disconnected from the control and/or regulating unit of the internal combustion engine and connected to diagnostic device  15 . This may be performed, for example, in that prior to the execution of method  71 , plug connector  13  between lambda sensor  11  and the control unit is manually disconnected and a plug connection is manually established between lambda sensor  11  and diagnostic device  15 . Method  71  is executed, for example, when the internal combustion engine is shut down or is at a stable operating point. Lambda sensor  11  may remain installed in the internal combustion engine in this case. However, it is also possible to remove lambda sensor  11  from the internal combustion engine prior to executing method  71 . Since neither diagnostic device  15  nor lambda sensor  11  is connected to the control unit of the internal combustion engine during the execution of method  71 , an isolated diagnosis of lambda sensor  11  may be carried out using method  71 . Interactions with the control and/or regulating unit of the internal combustion engine or other parts, in particular sensors and actuators of the internal combustion engine, may be at least largely precluded in this way. This is because the method is executed completely independently of the control and/or regulating unit of the internal combustion engine. 
         [0050]    After a start  73  of method  71 , a type of lambda sensor  11  is ascertained in a sensor recognition step  75 . Individual lambda sensors which are used in internal combustion engines for motor vehicles have substantially differing geometries, in particular of individual cells  17 ,  25 ,  63 , even if they have an identical design in principle (single-cell sensor or dual-cell sensor). Significant differences result therefrom with respect to the electrical properties of lambda sensors  11  of the various types. Sensor recognition step  75  ascertains the type of the sensor by electrical measurements, so that the subsequent steps of method  71  may be executed as a function of the ascertained type of lambda sensor  11 . 
         [0051]    In a step  76 , control unit  43  sets regulating element  59  in such a way that it regulates a temperature of the sensor element of the lambda sensor to a predefined setpoint value. In this case, an internal resistance of Nernst cell  25  or combined pump and Nernst cell  63 , which is a function of the temperature of the sensor element, is used as the control variable. A heating power of heating element  33 , which regulating element  59  may influence by changing heating voltage U H , for example, is used as the manipulated variable. Control unit  43  ascertains a setpoint value for the internal resistance, which it then predefines to regulating element  59 , from the predefined setpoint value of the temperature and a type of the lambda sensor identified in step  75 . As a function of the precise embodiment of diagnostic device  15 , the setpoint value of the temperature of the sensor element may either be predefined as a constant, or the setpoint value of the temperature may be predefined as a function of the type of the lambda sensor. It may also be provided that control unit  43  ascertains the setpoint value of the internal resistance directly as a function of the type of lambda sensor  11 , for example, on the basis of a table stored in control unit  43 . 
         [0052]    It is subsequently checked in a step  77  whether trim resistor  39  is correctly connected to terminals APE and RT. For this purpose, control unit  43  activates first switch element  47  and second switch element  49  in such a way that only first switch element  47  is closed. Furthermore, the control unit activates first voltage source  41  in such a way that a predetermined voltage U RT  is applied to voltage source  41  and therefore also between terminals APE and RT. Subsequently, control unit  43  detects current I 1  with the aid of first current sensor  45 , which corresponds to a current through trim resistor  39  in the case of intact lambda sensor  11 . 
         [0053]    Subsequently, it is checked in a branch  79  whether current I RT  is within a range delimited by a minimum value I RT,min  and a maximum value I RT,max . If this is not the case (N), an error is established in a step  81 . Control unit  43  may establish and/or log the error in step  81 . If the current is less than minimum value I RT,min , a bad contact of trim resistor  39  or an interruption between a terminal of trim resistor  39  and one of terminals APE or RT of plug connector  13  is recognized. If detected current I RT  is greater than maximum value I RT,max , a shunt parallel to trim resistor  39  is recognized. If the detected current is within the permissible range (Y), the sequence branches to a next test step  83 . Notwithstanding the specific embodiment shown, the resistance between terminals APE and RT may initially be calculated as a function of predefined voltage U RT  and detected current I RT  and the calculated resistance may be compared with a permissible resistance range. As a function of this comparison, a bad contact or interruption or a shunt may again be inferred in step  81 . 
         [0054]    Subsequently, a hysteresis within a relationship between a positive pump voltage U P &gt;0 and a pump current I P  is checked (see  FIG. 4 ). For this purpose, in step  83  a constant voltage U 1 =U P &gt;0 is applied by voltage source  41  to terminal APE and via closed second switch element  49  to terminal IPN of pump cell  17 . In the case of single-cell sensor  61 , voltage U P  is applied to terminals ALE and IPE. 
         [0055]    The value of pump voltage U P  is varied step-by-step. Initially, no voltage or only a low voltage is applied to pump cell  17  or pump and Nernst cell  63 , a relatively small value U P   1 , which may be 800 mV, for example, is then applied, and an associated current I P   1  is measured with the aid of first current sensor  45 . A higher pump voltage U P   2 , which may be 1200 mV, for example, is subsequently applied to terminals APE and IPN or ALE and IPE, and an associated current I P   2  is measured. After a certain time, smaller pump voltage U P   1  is again applied and an associated current I P   3  is detected. Subsequently, a branch  85  checks whether both current values I P   1  and I P   2  are zero. If this is the case (Y), a defect of the lines between terminal APE and/or IPN or ALE and/or IPE and cells  17  or  63  is recognized in a step  87 . Otherwise (N), it is checked in a branch  89  whether the absolute value of a difference between currents I P   3  and I P   1  is greater than a maximum value ΔI P,max . If this is the case (Y), a defect on at least one of electrodes  19 ,  21  or  65 ,  67  of cells  17  or  63  is recognized in a step  91 . If the difference between the currents is less than maximum value ΔI P,max , (branch N of branch  89 ), the hysteresis is sufficiently small and the sequence branches to a step  93 . 
         [0056]    In addition, in the case of a dual-cell broadband sensor, during both pump voltages U P   1  and U P   2  having the Ip, the Nernst voltages U N   1  and U N   2  between IPN and RE may also be measured. Both their absolute values and also the difference between them may be used as a diagnostic criterion. The sensitivity to defective IPN is thus improved and, in combination with the results of the Ip hysteresis study, an unambiguous differentiation as to which of the two pump electrodes is defective is made possible. 
         [0057]    In the following steps of method  71 , which are shown in  FIG. 5 , it is checked whether pump current I P  is in a permissible range. For this purpose, in a step  93 , a defined constant voltage U RT2  is initially applied by appropriate activation of first voltage source  41  and switch elements  47  and  49  and current I 1  is detected as a current I RT2 . A setpoint value I P,setpoint  for the pump current is ascertained from detected current I RT2  (step  95 ). Subsequently, a predefined constant positive pump voltage U P &gt;0 is applied to terminals APE and IPN in a step  97 . For this purpose, control unit  43  activates switch elements  47 ,  49  and first voltage source  41  appropriately (U 1 =U P &gt;0). Resulting pump current I P  is detected with the aid of first current sensor  45 . 
         [0058]    Subsequently, it is checked in a branch  99  whether the absolute value of a quotient of detected pump current I P  and ascertained setpoint value I P,setpoint  is in a range delimited by values Q min  and Q max . If this is not the case (N), an error in pump cell  17  is recognized in a step  101 . If the quotient is greater than value Q max , a crack is recognized in a diffusion barrier of lambda sensor  11  and/or in a sensor ceramic, in particular in first solid-state electrolyte  23 . Furthermore, an excessively large value of the quotient indicates an electrical shunt parallel to pump cell  17 . If the absolute value of the quotient is less than value Q min , sooting, i.e., dirt deposits, on the diffusion barrier is recognized. If the quotient is within the permissible range, the sequence continues with a step  103 . The precise value of Q min  or Q max  may be established as a function of the lambda sensor to be checked and the gas present at the sensor during the diagnosis. For specific types of lambda sensor  11  and specific gas environments, e.g., air, the quotient may deviate upward by up to 14%, i.e., Q max =1.14, for example. Correspondingly, a deviation by 14% downward may optionally also be tolerated, i.e., Q min =0.86 may be selected, for example. 
         [0059]    It is conceivable that steps  93 ,  95 ,  97 ,  99 ,  101  shown in  FIG. 5  for checking the hysteresis are also executed in the case of single-cell sensors and/or sensors without trim resistor  39 . In single-cell sensors, pump voltage U P  is applied to terminals ALE and IPE in step  97 . In exhaust gas sensors without trim resistor  39 , step  93  is omitted, and in step  95 , setpoint value I P,setpoint  of the pump current is established in another way, for example, as a constant which may optionally be a function of the type of the lambda sensor. 
         [0060]    Furthermore, in method  71 , in addition to pump current I P  in a forward direction, an inverted pump current is also checked. Corresponding steps of method  71  are shown in  FIG. 6 . In a step  103 , a negative voltage −U Pn  is generated by first voltage source  41 , i.e., U 1 &lt;0. The negative voltage is applied to terminals APE and IPN or ALE and IPE. For this purpose, control unit  43  closes first switch element  47  and opens second switch element  49 . A pump current I P  is detected in the case of applied negative pump voltage −U Pn . 
         [0061]    A branch  105  following step  103  checks whether the absolute value of detected pump current I P  is within a range delimited by values I P,min  and I P,max . If this is not the case (N), an error in lambda sensor  11  is established in a step  107 . Otherwise (Y), the sequence branches to a step  109 . If detected pump current I P  is less than minimum value I P,min , sooting of a protective layer applied to outer pump electrode  19  or outer electrode  65 , an excessively low temperature of lambda sensor  11 , and/or a defect in first solid-state electrolyte  23  are recognized in step  107 . If the absolute value of detected pump current I P  is greater than maximum value I P,max , an excessively high temperature of lambda sensor  11  and/or an electrical shunt between outer pump electrode  19  and inner pump electrode  21  or outer electrode  65  and inner electrode  67  or damage or a lack of the protective layer are recognized. Such a shunt may originate, for example, due to deposits between electrodes  19  and  21  or  65  and  67  or inadequate insulation of electrodes  19 ,  21  or  65 ,  67  from one another. 
         [0062]    As a further check shown in  FIG. 7 , the electrical conductivity between terminal IPN or IPE and conductive housing part  37  is checked. For this purpose, in step  109 , a voltage U ge  is applied between terminal IPN or IPE and electrically conductive housing part  37 . Voltage U ge  may be positive, U ge &gt;0. For this purpose, control unit  43  of the diagnostic device shown in  FIG. 1  closes third switch element  55  and keeps fourth switch element  57  open. Furthermore, control unit  43  activates second voltage source  51  in such a way that it generates voltage U 2 =U ge . Current I 2  flowing through second voltage source  51  is detected as a housing current I ge =I 2 . It is subsequently checked in a branch  111  whether detected housing current I ge  is greater than a critical value I ge,krit . If this is the case (Y), a shunt is recognized between a sensor element of lambda sensor  11  and housing  35  in a step  113 . Such a shunt may originate from sooting of lambda sensor  11 , in particular from a soot deposit between the sensor element and an inner side of a protective tube of housing  35 . If housing current I ge  is not greater than critical value I ge,krit  (N), the sequence branches to a step  115 . In step  115 , the test results ascertained in the preceding steps are analyzed. For example, they may be displayed and/or stored. It is also conceivable that, in particular if all tests have not delivered error findings individually, a multidimensional feature spectrum is checked. This means that the tolerance ranges of each individual studied functional variable are linked in a final test to where each of the other functional values lie. A more sensitive overall diagnosis may thus be achieved and interactions between individual parameters may also be considered. Subsequently, the method is terminated in step  117 . 
         [0063]    In the shown specific embodiment of method  71 , for the case in which an individual check recognizes an error, i.e., for the case in which one of steps  81 ,  87 ,  91 ,  101 ,  107 , or  113  is executed, method  71  is continued in each case using the next check. This means that all checks are performed independently of the results of the particular preceding check. In this case, control unit  43  controls the sequence of method  71  and analyzes detected variables for the diagnosis of lambda sensor  11 ,  61 . Control unit  43  therefore also forms an analysis unit of diagnostic device  15 . 
         [0064]    However, as a deviation it may also be provided that method  71  is terminated as soon as one of the checks recognizes an error. In this case, after execution of steps  81 ,  87 ,  91 ,  101 , or  107 , the sequence immediately branches to step  115 . The sequence of the particular checks shown in  FIGS. 3 through 7  may be varied arbitrarily. In other specific embodiments, these checks may also be individually omitted. 
         [0065]    Step  75  for recognizing the type of lambda sensor  11  is explained in greater detail hereafter on the basis of  FIG. 8 . In step  75 , control unit  43  initially activates regulating element  59  in such a way that a heating voltage U H  is applied between terminal H+ and terminal H− of lambda sensor  11  (step  121 ). An exact regulation of the temperature of lambda sensor  11  is not necessary for recognizing the type of lambda sensor  11 . Heating voltage U H  must only be sufficiently high so that for all sensor types, with which diagnostic device  15  is to be operated, a sufficiently high temperature of solid-state electrolytes  23 ,  27  is achieved at which solid-state electrolytes  23 ,  27  are able to conduct oxygen ions. In a subsequent step  123 , oxygen ions are transported to the diffusion gap of lambda sensor  11 . If lambda sensor  11  is a dual-cell sensor, a negative voltage U D &lt;0 is applied to Nernst cell  25 . For this purpose, control unit  43  activates second voltage source  51  in such a way that voltage U 2  has a negative value, i.e., U 2 =U D &lt;0. After a certain time, Nernst cell  25  is again disconnected from voltage U D . For this purpose, control unit  43  may open fourth switch element  57 . 
         [0066]    In a step  125  following step  123 , a voltage U M  between terminal APE and terminal RE, i.e., essentially a voltage between outer pump electrode  19  and reference electrode  31 , is detected with the aid of voltage sensor  52 . The level of voltage U M  is a measure for the oxygen content in the gas which is present on the side of lambda sensor  11  which faces toward outer pump electrode  19 . If this is oxygen-poor gas, a relatively high value results for voltage U M , which is typically greater than 450 mV. Steps  123  and  125  are therefore used to recognize oxygen-poor gas (rich gas recognition). Oxygen-poor gas may be present in particular if lambda sensor  11  remains installed in the exhaust pipe of the internal combustion engine during the diagnosis. This is because residual exhaust gas often remains in the exhaust pipe, which has a relatively low oxygen content, after a shutdown of the internal combustion engine for the purpose of diagnosis. Since the exhaust system, in particular the exhaust pipe in which lambda sensor  11  is installed, is sealed relatively well in relation to the ambient air in modern internal combustion engines, the oxygen content in the exhaust pipe only rises slightly at most, even after a longer shutdown of the internal combustion engine. 
         [0067]    If lambda sensor  11  is single-cell sensor  61 , voltage U D  is applied to terminals ALE and IPE in step  123 . For this purpose, control unit  43  activates first voltage source  41  in such a way that it generates positive voltage U D &gt;0, i.e., U 1 =U D &gt;0. In step  125 , voltage U M  between terminals ALE and IPE is measured with the aid of voltage sensor  52 . 
         [0068]    Subsequently, it is checked in a branch  127  whether the absolute value of detected voltage U M  is greater than a critical value U M,krit . If this is the case (Y), it is recognized that oxygen-poor, i.e., rich, gas is present, and a correction value ΔU is set to a value which corresponds to the absolute value of voltage U M  in a step  129 . Otherwise (N), correction value ΔU is set to zero in a step  131 . 
         [0069]    Step  129  or  131  is followed by a step  133 , in which a negative voltage is applied between terminal IPN and terminal RE. The absolute value of this voltage corresponds to a predetermined value U SD1 &gt;0, which is corrected by correction value ΔU&gt;0, i.e., second voltage source  51  generates voltage U 2 =−U SD1 −ΔU&lt;0. Current I 2  is simultaneously detected as pump current I SD2 . Subsequently, a positive predefined constant voltage U SD2 &gt;0 is applied to terminals IPN and RE in a step  135 , whereby the polarity of the voltage at terminals IPN and RE is reversed. Current I 2  is simultaneously detected as further pump current I SD2 . Finally, in a step  137 , the type of lambda sensor  11  is ascertained as a function of both detected pump currents I SD1  and I SD2 . After completing step  137 , method  71  continues with step  76 , which follows step  75 . 
         [0070]    For example, two types of lambda sensor  11  may be differentiated on the basis of detected pump currents I SD1  and I SD2  which differ with respect to their geometry, in particular the size of the air diffusion duct to reference electrode  31  or the size and location of Nernst electrode  29 . 
         [0071]    Due to the different geometries, a resistance of Nernst cell  25  of lambda sensor  11  of these different types is therefore different. A relatively great value for detected currents I SD1  and I SD2  therefore results in lambda sensor  11  of the type in which Nernst cell  25  has a small ohmic resistance and an open reference air duct. In lambda sensor  11  of the type in which the ohmic resistance of Nernst cell  25  is relatively large and the diffusion coefficient of the reference air duct is relatively small, these detected currents I SD1  and I SD2  are relatively small. It may thus be provided that in step  137 , the type of lambda sensor  11 , in which the resistance of Nernst cell  25  and the reference air duct is small, is recognized if the detected currents are both greater than specific predefined minimum values, i.e., if I SD1 &gt;X 1  and I SD2 &gt;X 2 . Correspondingly, it may be provided in step  137  that the type of lambda sensor  11 , in which the resistance of Nernst cell  25  and of the reference air duct is high, is recognized if the detected currents are less than specific predefined minimum values, i.e., if I SD1 &lt;Y 1  and I SD2 &lt;Y 2 . Other, contrary combinations between the detected currents may also characterize a sensor type. 
         [0072]    Above-described steps  133 ,  135 ,  137  for differentiating types of lambda sensor  11  may also be applied accordingly in connection with single-cell lambda sensor  61 . 
         [0073]    It is recognized that the above-described check is redundant in that two detected currents are checked in order to differentiate between two different types of the lambda sensor. This allows a particularly reliable differentiation of the types of lambda sensor  11 . In the cases in which the type of lambda sensor  11  cannot be unambiguously identified, method  71  may either be aborted or a user of diagnostic device  15  may be requested to manually input the type of lambda sensor  11 . In step  137 , it is established that the type of lambda sensor  11  cannot be unambiguously identified if neither of the two above-mentioned conditions with respect to currents I SD1  and I SD2  apply. In this way, incorrect identification of the type of lambda sensor  11  is avoided, for example, if the resistance of Nernst cell  25  has changed due to wear or aging effects (so-called dynamic effects) of lambda sensor  11 . 
         [0074]    Overall, the exemplary embodiments and/or exemplary methods of the present invention provide a method and a diagnostic device, using which a detail check of an exhaust gas sensor, in particular a lambda sensor, is possible, the check being able to be performed isolated from other components of the internal combustion engine, which may be when the internal combustion engine is shut down. In this way, effects corrupting the check as a result of interactions between various components of the internal combustion engine are at least largely eliminated. Easy operation of diagnostic device  15  is achieved by the automatic recognition of the type of lambda sensor  11 .