Abstract:
The invention provides a deterioration detector for an exhaust gas sensor capable of detecting an amount of change with age of an internal resistance and conducting deterioration judgment taking into consideration fluctuation between products and temperature dependency of the internal resistance of the exhaust gas sensor.  
     The deterioration detector for an exhaust gas sensor computes and stores an initial internal resistance of an exhaust gas sensor ( 5 ) and an electric heater ( 4 ) within a period of time determined by a data collection period judgement apparatus ( 109 ), learns and stores a correlation between the initial internal resistances of the exhaust gas sensor and the electric heater, after a data collection period of time, by an abnormal condition judgement apparatus ( 115 ), computes and stores a current internal resistance of the exhaust gas sensor ( 5 ) and the electric heater ( 4 ), and detects an abnormal condition in the case of the current internal resistance being greatly varied over a predetermined permissible value by comparing the current internal resistance of the exhaust gas sensor and the electric heater with the initial internal resistance learned and stored.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a deterioration detector for an exhaust gas sensor to detect a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine and, more particularly, to a deterioration detector for an exhaust gas sensor capable of totally detecting deterioration of an exhaust gas sensor and abnormal condition of an electric heater wherein an exhaust gas sensor used in combination with an electric heater thereby improving accuracy in detection of an exhaust gas is employed. The invention relates also to a method for detecting the deterioration.  
           [0003]    2. Background Art  
           [0004]    A conventional technique has been well known in which a concentration of oxygen contained in exhaust gas discharged from an internal combustion engine is detected, feedback control of an air/fuel ratio of fluid mixture supplied to the internal combustion engine is conducted and exhaust gas is purified and a rate of fuel combustion is improved.  
           [0005]    In order to conduct the feedback control, it is necessary to keep the exhaust gas sensor within an activating region and stabilize detection characteristic of the oxygen concentration. Therefore, a ceramic heater incorporated in the exhaust gas sensor is controlled in terms of energizing and the exhaust gas sensor is kept at a constant temperature.  
           [0006]    As to detection of a temperature of an exhaust gas sensor to serve as a basic art, for example as disclosed in the Japanese Patent Publication (unexamined) No. 292364/1997 and the Japanese Patent Publication (unexamined) No. 313477/1996, a method is employed wherein an internal resistance of an exhaust gas sensor is measured and temperature of the exhaust gas sensor itself is detected using the internal resistance of the exhaust gas sensor as a reference. Additionally, as disclosed in the Japanese patent Publication (unexamined) No. 313476/1996 and the Japanese patent publication (unexamined) No. 4502/1997, such a method is employed as to detect a peripheral temperature of an exhaust gas sensor by measuring a resistance value of an electric heater incorporated in the exhaust gas sensor.  
           [0007]    As for a method of detecting deterioration of an exhaust gas sensor, as disclosed in the Japanese Patent Publication (unexamined) No. 331700/1998 and the Japanese Patent Publication (unexamined) No. 121221/1996, means for changing intentionally operating condition of an internal combustion engine and observing reaction change speed and output variance waveform of a detection output of an exhaust gas sensor in response to the change of operating condition of the internal combustion engine.  
           [0008]    In case of employing such a conventional art as described above, a problem exists in that large-scaled means for changing operating condition of the internal combustion engine has been required just for detecting the deterioration of the exhaust gas sensor. A further problem exists in that abnormal condition of the electric heater integrated with the exhaust gas sensor cannot be detected.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention was made to solve the above-discussed problems, and has an object of providing a deterioration detector for an exhaust gas sensor and a method for detecting deterioration wherein total deterioration detection of an exhaust gas sensor including deterioration of an electric heater is easily performed simply by utilizing means for observing change with age in internal resistance of an exhaust gas sensor together with detection of abnormal condition of the electric heater.  
           [0010]    The present invention is to provide a simple deterioration detector for an exhaust gas sensor and a method for detecting deterioration wherein change with age in internal resistance is extracted taking into consideration fluctuation between products and temperature dependency of an exhaust gas sensor and an electric heater.  
           [0011]    A deterioration detector for an exhaust gas sensor according to the present invention that is mounted on an exhaust pipe of an internal combustion engine for an automobile and is arranged so as to control a temperature by an electric heater thereby detecting deterioration of the exhaust gas sensor comprises:  
           [0012]    initial internal resistance correlation learning storage means for sequentially sampling, comparing, computing and storing an initial internal resistance of said exhaust gas sensor and said electric heater;  
           [0013]    data collection period judgement means for determining a period of sampling said initial internal resistance; and  
           [0014]    abnormal condition judgment means for computing and storing a current internal resistance of said exhaust gas sensor and said electric heater after a predetermined period of time of an operation start of said internal combustion engine for an automobile, for judging that said current internal resistance has varied greatly over a predetermined permissible value by comparing said current internal resistance with said initial internal resistance, thereby conducting a processing for the abnormal condition.  
           [0015]    As a result of such an arrangement, deterioration of the exhaust gas sensor as well as the abnormal condition of the electric heater can be detected, and a timing of replacement and repair of an integrally formed exhaust gas sensor and electric heater can be known.  
           [0016]    In the deterioration detector for an exhaust gas sensor of above arrangement according to the invention, it is preferable that, in the case that an initial internal resistance of the exhaust gas sensor and the electric heater which is newly sampled, computed and measured is closely analogous to the initial internal resistance already learned and stored, the initial internal resistance correlation learning storage means averages the value of said initial internal resistance newly sampled and that of said initial internal resistance learned and stored, and stores the averaged value for renewal.  
           [0017]    As a result of such arrangement, any useless proximate information is not stored, a memory capacity can be reduced, and an amount of information to be referenced and read out is eliminated, making it possible to perform a high-speed processing.  
           [0018]    Further, in a deterioration detector for an exhaust gas sensor of above arrangement according to the invention, it is preferable that the data collection period judgement means judges a completion of the data collection period based on the fact that a sampling number of the initial internal resistance of the exhaust gas sensor or the electric heater stored in initial internal resistance correlation learning storage means, is not less than a predetermined number and a correlation coefficient not less than a predetermined value is surely obtained.  
           [0019]    As a result of such arrangement, even if there might be any fluctuation of the internal resistance of the exhaust gas sensor or the electric heater between one sensor or heater and another, learning and storing an initial value of an actually utilized sensor or heater enables to detect a later change, and a data collection period can be determined by confirming whether or not a sufficient number of data could be obtained.  
           [0020]    Further, in a deterioration detector for an exhaust gas sensor of above arrangement according to the invention, it is preferable that the abnormal condition judgment means comprises computing interpolation means for computing, on the assumption that either one of the current internal resistance of the exhaust gas sensor or the electric heater is an initial internal resistance, the other correlation initial internal resistance, comparing the other correlation initial internal resistance with the other current internal resistance, and producing an abnormal condition judgement output in the case of a difference between the other correlation initial internal resistance and the other current internal resistance being greatly varied over a predetermined permissible value.  
           [0021]    As a result of such arrangement, even if the internal resistance of the exhaust gas sensor and the electric heater has greatly changed due to an environmental temperature, it can be discriminated whether the current internal resistance has changed with age or has changed due to environmental temperature. An accurate correlation value can be calculated even though there might be a small amount of stored information, making it possible to reduce a memory capacity.  
           [0022]    It is also preferable that the deterioration detector for an exhaust gas sensor of above arrangement according to the invention, in addition to the arrangement as described above, further comprises reset means for initializing information stored in each of data collection period judgement means, initial internal resistance correlation learning storage means and abnormal condition judgement means in response to a reset signal from outside.  
           [0023]    As a result of such arrangement, even in the case of replacing the deteriorated exhaust gas sensor with a new one, detection of deterioration of such a new exhaust gas sensor can be conducted under the condition suitable for the new exhaust gas sensor.  
           [0024]    A method for detecting deterioration of an exhaust gas sensor which is mounted on an exhaust pipe of an internal combustion engine for an automobile and arranged so as to control a temperature by an electric heater thereby detecting the deterioration of the exhaust gas sensor, comprising the steps of:  
           [0025]    computing and storing an initial internal resistance of said exhaust gas sensor and said electric heater;  
           [0026]    learning and storing a correlation in initial internal resistance between said exhaust gas sensor and said electric heater;  
           [0027]    computing and storing a current internal resistance of said exhaust gas sensor and said electric heater;  
           [0028]    calculating, on the assumption that either one of the current internal resistance of said exhaust gas sensor or said electric heater is an initial internal resistance based on said correlation learned and stored, the other logical internal resistance; and  
           [0029]    detecting a deterioration condition of said exhaust gas sensor and said electric heater by comparing the logical internal resistance with the other current internal resistance.  
           [0030]    As a result, deterioration of the exhaust gas sensor as well as the abnormal condition of the electric heater can be detected, and a timing of replacement and repair of an integrally formed exhaust gas sensor and electric heater can be known.  
           [0031]    Furthermore, even if the internal resistance of the exhaust gas sensor and the electric heater has greatly changed due to an environmental temperature, it can be discriminated whether the current internal resistance has changed with age or has changed due to environmental temperature. An accurate correlation value can be calculated even though there might be a small amount of stored information, making it possible to reduce a memory capacity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a circuit diagram of a deterioration detector for an exhaust gas sensor according to an embodiment of the present invention.  
         [0033]    [0033]FIG. 2 is a flow chart showing an operation of the deterioration detector for the exhaust gas sensor according to the embodiment  1  of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    A deterioration detector for an exhaust gas sensor according to an embodiment of the present invention is hereinafter described with reference to FIG. 1 and FIG. 2. FIG. 1 is an electric circuit diagram showing mainly an electric circuit of a deterioration detector for an exhaust gas sensor, and FIG. 2 is a flow chart showing an operation of the deterioration detector for an exhaust gas sensor arranged as shown in FIG. 1. In FIG. 1, reference numeral  1  is a deterioration detector to which an electrical power is supplied from a vehicle-mounted battery  2  of, for example, 12 volt via a power supply switch  3 . Reference numeral  2   a  is a vehicle body to which a negative terminal of the vehicle-mounted battery  2  is connected. Numeral  4  is an electric heater that is electrically connected to the deterioration detector  1  via terminals  4   a  and  4   b . Numeral  5  is an exhaust gas sensor that is electrically connected via a terminal to the deterioration detector  1  attached to an exhaust pipe of an internal combustion engine for an automobile. Numeral  6  is an internal resistance of the exhaust gas sensor  5 , and the electric heater  4  and exhaust gas sensor  5  form an integral construction disposed adjacent to each other. Further, the electric heater  4  conducts temperature control of the exhaust gas sensor  5 .  
         [0035]    In addition, the deterioration detector  1  not only detects the deterioration of the exhaust gas sensor  5  and abnormal condition of the electric heater  4  but also is constructed to include various control functions such as the temperature control of the exhaust gas sensor  5  by the electric heater  4 . Numeral  7  is a reset input switch that is connected to the deterioration detector  1 . Numeral  7   a  is a data set system that is connected to the deterioration detector  1  and conducts various data setting upon delivery of a product and replacement of parts. Numeral  8  is an alarm display element such as light emitting diode that is connected to the deterioration detector  1  via a current limiting resistance  9 .  
         [0036]    Now, an internal arrangement of the deterioration detector  1  is hereinafter described.  
         [0037]    Reference numeral  10  is a positive power supply wire that is connected to the vehicle-mounted battery  2  via the power supply switch  3 . Numeral  11  is for generating a constant voltage output, for example, of DC 5 V, and this constant voltage power supply system  11  is connected to the power supply wire  10 . Numeral  12  is a microprocessor to which power is supplied from the constant power supply system  11 . Further, one terminal  4   a  of the electric heater  4  is connected to the power supply wire  10 .  
         [0038]    Numerals R 1  and R 2  are voltage dividing resistances connected to the power supply wire  10 . A voltage dividing value of the power supply voltage is connected to an input terminal AD 1  of the microprocessor  12  for conversion from analog to digital thereby measuring the power supply voltage.  
         [0039]    Numeral AMP  1  is an amplifier that is connected between the exhaust gas sensor  5  and an input terminal AD 2  of the microprocessor  12  for conversion from analog to digital. Numeral TR 1  is a transistor for connecting a load resistance R 3  to the exhaust gas sensor  5 . R 4  is a base resistance that is connected to a pulse output terminal DR 1  of the microprocessor  12  and drives the transistor TR 1  to periodically turn ON/OFF. R 5  is a ballast resistance in order to cause the transistor  1  to surely turn OFF.  
         [0040]    Numeral TR 2  is an open/close element such as a transistor, which is connected to the other terminal  4   b  of the electric heater  4 . Numeral R 6  is a current detection resistance that is connected in series to the open/close element TR 2 . Numeral R 7  is a base resistance that is connected to a pulse output terminal DR 2  of the microprocessor  12  and drives the open/close element TR 2  to periodically turn ON/OFF. Numeral R 8  is a ballast resistance that causes the open/close element TR 2  to surely turn OFF.  
         [0041]    Numerals R 9  and R 10  are voltage-dividing resistances that are connected putting the open/close element TR 2  between them. Numeral MP 2  is an amplifier for amplifying the divided voltage by the voltage dividing resistances and supplying the amplified voltage to a conversion input terminal AD 3  of the microprocessor  12  for conversion from analog to digital. Distribution of resistance values such as voltage dividing resistances is as follow: 
           Rh&gt;&gt;Re R 9&gt;&gt; Rh   (1) 
           Rh:Re≈R 9: R 10  (2) 
         [0042]    where: Rh shows an internal resistance of the electric heater  4 , Re shows a resistance value of a current detection resistance R  6 , R 9  shows a resistance value of the voltage dividing resistance R 9 , and R 10  shows a resistance value of the voltage dividing resistance R 10  respectively.  
         [0043]    Numeral DR 3  is an output terminal of the microprocessor  12 , and this microprocessor  12  drives the alarm display element  8  such as a light emitting diode via a current-limiting resistance  9 . RST is an input terminal of the microprocessor  12  that is connected to the reset input switch  7 . R/W is a terminal for serial communication of the microprocessor  12  that is connected to the data set system  7   a , and the data set system  7   a  is connected to the terminal R/W for a serial communication only in a specific condition such as product delivery adjustment or parts replacement maintenance work.  
         [0044]    Now, an operation of a deterioration detector for an exhaust gas sensor having an electric circuit arranged as shown in FIG. 1 is described with reference to a flow chart of FIG. 2.  
         [0045]    In FIG. 2, Step  101  is an operation start step wherein an operation of a deterioration detector for an exhaust gas sensor starts just after a start-up of an engine of an automobile. After Step  101 , a program proceeds to Step  102 , wherein a reset mode judgement is conducted. In Step  102  (reset mode judgement step), it is judged to be a reset mode when the reset input switch  7  is ON, or a reset signal from the data set system  7   a  is input.  
         [0046]    In Step  102 , when it is judged not to be a reset mode, the program proceeds to Step  103  wherein a data collection completion is judged. In Step  103 , it is judged whether a correlation coefficient computed and stored in later described Step  104  (correlation coefficient computing storage means) has reached to a predetermined value. The correlation coefficient is one data to be computed, renewed and come close to 1.0 sequentially, and data collection is judged completed when the value reaches not less than, for example, 0.7 (a predetermined value).  
         [0047]    In Step  103 , in the case that it is judged that data collection has not completed, the program proceeds to Step  104 . In Step  104 , computed and stored is a correlation coefficient of a pair of initial internal resistances of the exhaust gas sensor  5  versus the electric heater  4  to be stored in Step  107  later described wherein a correlation table of the initial internal resistances is prepared and stored (corresponding to initial internal resistance correlation learning storage means). The Step  104  is conducted when a sampling number of the initial internal resistances stored in later described Step  107  reaching not less than a predetermined number.  
         [0048]    Step  109  corresponds to data collection period judgment means wherein Step  103  and Step  104  are included. As described above, in data collection period judgment means  109 , based on the fact that a sampling number of an initial internal resistance of the exhaust gas sensor or the electric heater stored in initial internal resistance correlation learning storage means (corresponding to Step  107 ) is not less than a predetermined value and that a correlation coefficient not less than a predetermined value is secured, it is judged that data collection period has completed.  
         [0049]    Then after Step  104 , the program proceeds to Step  105  (initial internal resistance computing storage means). In Step  105 , the internal resistance of the exhaust gas sensor is computed and stored. In Step  105 , as shown in FIG. 1, when DR 1  terminal of the microprocessor  12  comes to be a logical level L and a transistor TR 1  is OFF, a generated voltage ES of the exhaust gas sensor  5  is directly input via AMP 1  to an AD 2  terminal of the microprocessor  12 .  
         [0050]    Then, when DR 1  terminal comes to be a logical level H and the transistor TR 1  is ON, a divided voltage E of the exhaust gas sensor  5  is input via AMP 1  to AD 2  terminal of the microprocessor  12 .  
         [0051]    Consequently, a resistance value RS of the internal resistance  6  of the exhaust gas sensor  5  is calculated by the following equation. 
           E=ES×R 3/( R 3+ RS )∴ RS=R 3×( ES/E− 1)  (3) 
         [0052]    where: R 3  is a resistance value of a load resistance R 3 .  
         [0053]    In addition, although an actual value of resistance value RS of the internal resistance  6  of the exhaust gas sensor  5  is greatly different depending on a type of the exhaust gas sensor  5 . For example, a liner-type oxygen concentration sensor has an extremely large temperature dependency such as 90Ω→30Ω corresponding to an atmosphere temperature 600° C.→700° C.  
         [0054]    Subsequent to Step  5 , the program proceeds to Step  6  (initial internal resistance computing storage means), in which the internal resistance of the electric heater  4  is computed and stored. In this Step  6 , as shown in FIG. 1, when the DR 2  terminal comes to be a logical level L and the open/close element TR 2  is OFF, a divided voltage Eoff is input via AMP 2  to AD 3  terminal of the microprocessor  12 . The divided voltage Eoff is expressed in the following equation. 
           Eoff=Eb× ( R 6+ R 10/( Rh+R 9+ R 10+ R 6)  (4) 
         [0055]    where: Eb is a power supply voltage which is measured by AD 1  terminal of the microprocessor  12 , R 6  is a resistance value of the current detection resistance  6 , R 9  and R 10  are resistance values of the voltage dividing resistances R 9  and R 10  respectively, and Rh is an internal resistance value of the electric heater  4 .  
         [0056]    A value of internal resistance Rh of the electric heater  4  calculated by equation(4), has a temperature dependency, for example, 21.5Ω→23Ωcorresponding to an atmosphere temperature 600° C.→700° C.  
         [0057]    Further, when DR 2  terminal of the microprocessor  12  comes to be logical level H and the open/close element TR 2  is ON, based on the fact that the divided voltage Eon is input to AD 3  terminal of the microprocessor  12  via AMP 2 , an internal resistance of the electric heater  4  can be calculated by the following equation. 
           Eon≈Eb×R 6/( Rh+R 6)  (5) 
         [0058]    However, there has been a temperature difference generated between a temperature of the electric heater  4  and an environmental temperature of the exhaust gas sensor  5 , it is preferable to measure an internal resistance in the condition that the electric heater  4  has not been energized for not less than a predetermined period of time.  
         [0059]    Further, in the case that an initial internal resistance of the exhaust gas sensor and electric heater which is newly sampled, computed and measured, is closely analogous to the initial internal resistance which is already learned and stored, in initial internal resistance correlation learning storage means (corresponding to Step  107 ), a value to be obtained by averaging the values of the initial internal resistance newly sampled and the initial internal resistance learned and stored, is renewed and stored. As a result, any useless proximate information is not stored so as to reduce a memory capacity, and an information amount to be referenced and read out is eliminated so that a high-speed processing of the microprocessor  12  can be conducted.  
         [0060]    Then after Step  106 , the program proceeds to Step  107 , and in this Step  107 , a correlation table of an initial internal resistance is prepared and stored (correlation table learning storage step). In Step  107 , prepared is a table of an internal resistance of the electric heater  4  Rhi=F (Ti) versus an internal resistance of the exhaust gas sensor  5  Rsi=G(Ti) under various environmental temperature Ti (i= 1 ,  2 , . . . n).  
         [0061]    However, note that an environmental temperature Ti itself as a parameter is an unknown value and is not stored in the table, and it is a table just to know a direct correlation Rsi=H (Rhi), and above described letter F, G, H show correlation symbol.  
         [0062]    Then after Step  107 , the program proceeds to Step  108 . Step  108  is an end step, and in which it is controlled so as to return to a start Step  101 , for example a several minutes later.  
         [0063]    Further, the internal resistances utilized in Step  105 -Step  107  are those of the exhaust gas sensor  5  or the electric heater  4  in an initial condition (generally during several months after start the use), and it is required that initial values of the exhaust gas sensor  5  and the electric heater  4  themselves practically connected and used be learned due to a large fluctuation between products.  
         [0064]    In the case that a completion of data collection is judged in Step  103 , the program proceeds to Step  110 . The Step  110  is a step wherein a current internal resistance Rs of the exhaust gas sensor  5  is calculated and stored, and the computing equation is as shown in the above equation (3).  
         [0065]    Then after Step  111 , the program proceeds to Step  112 . The Step  112  is a computing interpolation step. In Step  112 , by using a correlation learning value of an initial internal resistance Rsi=H (Rhi) prepared in Step  107 , a logical internal resistance with respect to a current internal resistance Rh measured in Step  111  RsO=H (Rh).  
         [0066]    However, when there is no value coincident to a current internal resistance Rh in Rhi (i= 1 ,  2 , . . . n), by being linearly interpolated from internal resistances at two points Rhj and Rhk which are values above and below an internal resistance Rh ( Rhj &lt;Rh &lt;Rhk ), a logical internal resistance Rso with respect to an internal resistance Rh is calculated.  
         [0067]    Then after Step  112 , the program proceeds to Step  113 . In Step  113 , an internal resistance Rs (current resistance) computed and measured in Step  110  and a logical internal resistance RsO (initial resistance) calculated in Step  112  are compared with each other, and it is judged whether or not a difference value is greatly varied over a predetermined permissible variation value (a predetermined permissible value).  
         [0068]    In Step  113 , in case of judging that there has been a greater variation than a predetermined permissible value, the program proceeds to Step  114 . The Step  114  is a step wherein an abnormal condition judgement output is generated, and it is arranged in such a manner that an alarm display element  8  is lighted by the abnormal condition judgement output.  
         [0069]    In the case that either operation of Step  114  has completed or Step  113  judges the condition normal (no great variation), the program proceeds to Step  108 , and it is externally controlled to proceed again to start Step  101   a  several minutes later. Numeral  115  is abnormal condition judgement means including Step  110  Step  113 .  
         [0070]    As described above, the abnormal condition judgment means  115  comprises a computing interpolation means (Step  112 ) for computing, on the assumption that either one of the current internal resistances of the exhaust gas sensor  5  or the electric heater  4  is an initial internal resistance, the other correlation initial internal resistance (logical internal resistance), compares the other correlation initial internal resistance with the other current internal resistance, and produces an abnormal condition judgement output in the case that a difference between the other correlation initial internal resistance and the other current internal resistance has varied greatly over a predetermined permissible value. Thus, when an abnormal condition is detected in either one or both of the exhaust gas sensor  5  and the electric heater  4 , it is possible to know a timing to replace any integrally formed parts of the exhaust gas sensor  5  and the electric sensor  4 .  
         [0071]    Furthermore, in the case of Step  102  being a reset mode, the program proceeds to Step  120 . In Step  120 , a computed and stored value of a correlation coefficient obtained by correlation coefficient computing storage means (Step  104 ) within the data collection period judgement means  109 , is reset and collection times are also reset.  
         [0072]    Then after Step  120 , the program proceeds to Step  121 . The Step  121  is a table-reset step, and reset is a stored correlation value of an initial internal resistance learned and stored in correlation table creation storage step (Step  107 ) Then after Step  121 , the program proceeds to Step  122 . The Step  122  is an abnormal condition alarm reset step, and by resetting an abnormal condition judgement output produced in abnormal condition judgement output means (Step  114 ), the program  5  proceeds to the end Step  108 . Further, numeral  123  is reset means including Step  102 , Step  120 -Step  122 .  
         [0073]    As described above, reset means  123  initializes a storage information stored respectively in each reset means  123 , data collection period judgement means  109 , initial internal resistance correlation learning storage means (corresponding to Step  107 ) and abnormal condition judgement means  115 , in response to a reset signal from the outside. As a result, based on the fact that the integrally formed exhaust gas sensor  5  and electric heater  4  are performed after replacement of parts, it becomes possible to detect deterioration corresponding to a characteristic of a newly incorporated exhaust gas sensor  5  and electric heater  4 .  
         [0074]    As described above, a deterioration detector for an exhaust gas sensor according to the invention comprises: initial internal resistance correlation learning storage means (corresponding to Step  107 ) for sequentially sampling, calculating and storing an initial internal resistance of the exhaust gas sensor  5  and the electric heater  4 ; data collection period judgement means  109  for determining a period of time of sampling the initial internal resistance; and abnormal condition judgement means  115  for computing and storing a current internal resistance of the exhaust gas sensor  5  and the electric heater  4  after a predetermined period of time following a start-up of an internal combustion engine for an automobile, and for conducting an abnormal condition processing in the case of the current internal resistance being greatly varied over a predetermined permissible value by comparing the current internal resistance with the initial internal resistance. As a result of employing such arrangement, observation of change with age of the exhaust gas sensor  5  enables the detection of deterioration of the exhaust gas sensor  5 . Furthermore, even if an abnormal condition such as a short circuit or an opened circuit in the electric heater  4  should take place, abnormal condition output is generated, and abnormal condition of the electric heater  4  can be also detected.  
         [0075]    In addition, an exhaust gas sensor shown in FIG. 1 is an example of a non-liner exhaust gas sensor, however it is preferable to employ such a type of exhaust gas sensor as to measure an internal resistance by utilizing a liner-type exhaust gas sensor having two or three terminals. It is also preferable that an integrated circuit parts such as those for obtaining a detection oxygen voltage signal or a voltage signal in proportion to the internal resistance be incorporated outside the microprocessor  12 .  
         [0076]    Additionally, in the foregoing description of FIG. 2, a correlation learning value of the initial internal resistance learned and stored in Step  107  is obtained by calculating an internal resistance Rsi of the exhaust gas sensor  5  employing an internal resistance Rhi of the electric heater  4  as a reference, however either of the mentioned internal resistances may be an available reference.