Abstract:
A fault diagnostic apparatus is provided that enables improving estimate accuracy of the temperature of an object to be examined and to improve fault diagnosis accuracy of a temperature detector for detecting a temperature of the object to be examined. The fault diagnostic apparatus is for diagnosing a fault of a water temperature sensor for detecting a temperature of cooling water and includes a fault diagnostic device for diagnosing a fault of the water temperature sensor based on a change in temperature detected by the water temperature sensor in a situation where a parameter to be a chief source of heating the cooling water (such as an engine temperature) decreases.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fault diagnostic apparatus and, more particularly, to a fault diagnostic apparatus for diagnosing a fault of a temperature sensor such as a water temperature sensor. 
   2. Description of the Relevant Art 
   Water and oil are useful for cooling an engine. The temperature of water or oil is one item of data for judging a state of the engine. For example, under idle speed control, an air flow rate is determined depending on the water temperature, and a corrective factor for correcting a basic fuel injection amount for the sake of improving a warm-up characteristic and an accelerating ability and cleaning emission gas is determined depending on the water temperature. 
   When this water temperature sensor for detecting a water temperature has a fault, control of each kind is carried out based on an abnormal output signal, and therefore, there is a possibility that some problems may be caused. In order to solve this problem, techniques of several kinds for diagnosing a fault of a water temperature sensor have been proposed. 
   For example, Japanese Patent Application Laid-Open No. 11-82144 discloses an invention wherein a water temperature sensor is judged as being in an abnormal condition when a cooling water temperature detected after an engine was judged as being warmed up is lower than a predetermined water temperature. Japanese Patent Application Laid-Open No. 2004-76689 discloses an invention wherein by estimating a cooling water temperature based on a parameter related to a heating value of an engine (an amount of heat to be transferred from the engine to the cooling water) and a parameter related to a quantity of radiant heat of the cooling water, and by comparing the estimated cooling water temperature with a cooling water temperature detected by a water temperature sensor, whether a flow regulating valve for regulating a flow rate of the cooling water is abnormal or not is checked. 
   However, in both of the inventions described in the Patent Documents 1 and 2, a fault of the water temperature sensor or the flow regulating valve is diagnosed with the engine running (i.e. in a situation where the temperature of the engine to be a chief source of heating the cooling water is high). When the temperature of the engine is high, the heating value from the engine also becomes high, leading to a rise in cooling water temperature. As a result, since the cooling water temperature varies widely under a great influence of the heating value from the engine, there is a possibility that a very high degree of estimate accuracy of the cooling water temperature cannot be expected while the engine is running. 
   SUMMARY OF THE INVENTION 
   The present invention was accomplished in order to solve the above problem, and it is an object of the present invention to provide a fault diagnostic apparatus, whereby estimate accuracy of temperature of an object to be examined such as cooling water can be improved and fault diagnosis accuracy of a temperature detector for detecting a temperature of the object to be examined can be improved. 
   In order to achieve the above object, a fault diagnostic apparatus according to a first aspect of the present invention is characterized by a fault diagnostic apparatus for diagnosing a fault of a temperature detector for detecting a temperature of an object to be examined, comprising a fault diagnostic device for diagnosing a fault of the temperature detector based on a change in temperature detected by the temperature detector in a situation where a parameter of a chief source of heating the object to be examined decreases. 
   When the fault diagnostic apparatus according to the first aspect of the present invention is used, an abnormal condition of the temperature detector is checked in a situation where a parameter of a chief source of heating the object to be examined (e.g. a temperature of a heat source) decreases. In the situation where the parameter decreases, the temperature of the object to be examined also falls gradually with the decrease. Therefore, by monitoring a change in temperature of the object to be examined, a fault of the temperature detector can be diagnosed. Since the parameter never rises, it becomes easier to estimate a temperature change of the object to be examined. Thus, the fault diagnosis accuracy of the temperature detector can be improved. 
   A fault diagnostic apparatus according to a second aspect of the present invention is characterized by a fault diagnostic apparatus for diagnosing a fault of a temperature detector for detecting a temperature of an object to be examined, wherein the object to be examined is mounted on a vehicle, the fault diagnostic apparatus comprising a fault diagnostic device for diagnosing a fault of the temperature detector based on a change in temperature detected by the temperature detector in a situation where a temperature of an internal combustion engine to be a chief source of heating the object to be examined decreases, wherein a case of an ignition (IG) switch being in the off position is judged as being in a situation where the temperature of the internal combustion engine decreases. 
   When the fault diagnostic apparatus according to the second aspect of the present invention is used, the object to be examined is mounted on a vehicle. Therefore, it is possible to prevent a problem in movement control of the vehicle from being caused by an occurrence of a fault of the temperature detector. 
   Moreover, whether the temperature detector is in an abnormal condition or not is checked in a situation where a temperature of an internal combustion engine (e.g. an engine) to be a chief source of heating the object to be examined decreases (e.g. an IG switch is in the off position). In the situation where the temperature of the internal combustion engine decreases, the temperature of the object to be examined also falls gradually with the decrease. As a result, it can be made easier to estimate a temperature change of the object to be examined. 
   Furthermore, when the fault diagnostic apparatus according to the second aspect of the present invention is used, a case where an IG switch is in the off position is judged as being in a situation where the temperature of the internal combustion engine decreases. As a result, whether or not to be is in a situation where the temperature of the internal combustion engine decreases can be appropriately judged. 
   A fault diagnostic apparatus according to a third aspect of the present invention is characterized by the fault diagnostic device, comprising a temperature estimating unit for estimating a temperature of the object to be examined based on parameters related to a quantity of radiant heat of the object to be examined and/or a heating value to the object to be examined, and a temperature comparing unit for comparing an estimated temperature estimated by the temperature estimating unit with a temperature detected by the temperature detector, wherein the fault diagnostic device diagnoses a fault of the temperature detector based on a comparison result by the temperature comparing unit in the fault diagnostic apparatus according to the second aspect of the present invention. 
   A temperature change of the object to be examined is determined depending on a quantity of radiant heat of the object to be examined and a heating value to the object to be examined. In other words, the temperature change of the object to be examined can be properly estimated by giving consideration to the quantity of radiant heat of the object to be examined and the heating value to the object to be examined. 
   When the fault diagnostic apparatus according to the third aspect of the present invention is used, a temperature of the object to be examined is estimated based on parameters related to a quantity of radiant heat of the object to be examined and/or a heating value to the object to be examined. An estimated temperature estimated in this way and a temperature detected by the temperature detector are compared with each other, and a fault of the temperature detector is checked based on this comparison result. As a result, since a fault of the temperature detector is diagnosed by comparing an estimated temperature properly estimated with a temperature detected by the temperature detector, the accuracy of the diagnosis can be improved. 
   A fault diagnostic apparatus according to a fourth aspect of the present invention is characterized by the temperature comparing unit which compares an estimated temperature at the end of a predetermined time period after the IG switch is turned off with a measured temperature detected by the temperature detector in the fault diagnostic apparatus according to the third aspect of the present invention. 
   When the fault diagnostic apparatus according to the fourth aspect of the present invention is used, an estimated temperature at the end of a predetermined time period after the IG switch is turned off (e.g. after an elapse of 180 minutes from a stop of the engine) and a measured temperature at that time are compared with each other. Thus, for example, by comparing an estimated temperature with a measured temperature after the engine to be a chief source of heating the object to be examined (cooling water for cooling the engine) is sufficiently cooled, a fault of the temperature detector (water temperature sensor) for detecting a temperature of the object to be examined can be diagnosed. 
   Concerning a temperature of the object to be examined, it appears that the detection accuracy thereof is enhanced better when the engine is in a low-temperature condition than in a high-temperature condition. Therefore, by comparing an estimated temperature with a measured temperature at the end of the predetermined time period after the IG switch is turned off, the accuracy of the fault diagnosis can be improved. 
   A fault diagnostic apparatus according to a fifth aspect of the present invention is characterized by the temperature estimating unit which estimates a temperature of the object to be examined after an elapse of a predetermined time period from a prescribed time based on a map showing relationships among a temperature of the object to be examined at the prescribed time, a parameter at the prescribed time, a parameter after an elapse of the predetermined time period from the prescribed time and a temperature of the object to be examined that drops in the predetermined time period after the prescribed time in the fault diagnostic apparatus according to the third or fourth aspect of the present invention. 
   When the fault diagnostic apparatus according to the fifth aspect of the present invention is used, by detecting a temperature of the object to be examined at the prescribed time, a parameter at the prescribed time and a parameter after an elapse of a predetermined time period from the prescribed time, a temperature of the object to be examined after an elapse of the predetermined time period form the prescribed time can be properly estimated. 
   For example, when the below-mentioned pieces of information a-c are detected, a temperature of cooling water after an elapse of 180 minutes from a stop of the engine can be estimated. 
   a. A temperature of the cooling water at the stop of the engine 
   b. An outside air temperature at the stop of the engine 
   c. An outside air temperature after an elapse of 180 minutes from the stop of the engine 
   A fault diagnostic apparatus according to a sixth aspect of the present invention is characterized by the temperature estimating unit which estimates a temperature of the object to be examined after an elapse of a predetermined time period based on a temperature of the object to be examined detected by the temperature detector at a prescribed time and, a parameter at the prescribed time or a parameter after an elapse of the predetermined time period from the prescribed time, or a parameter at the prescribed time and a parameter after an elapse of the predetermined time period from the prescribed time in the fault diagnostic apparatus according to the third or fourth aspect of the present invention. 
   In the case of estimating a temperature change of the object to be examined in an elapse of time a after time A, the below-mentioned pieces of information a-c make the estimate accuracy higher. 
   a. A temperature of the object to be examined at the time A 
   b. An environment (e.g. an outside air temperature) at the time A 
   c. An environment (e.g. an outside air temperature) after an elapse of the time a after the time A 
   However, if the time a is not so long, the environment of c may be substituted with the environment of b, or vice versa. For example, it may be adoptable to think that the outside air temperature does not change in an elapse of the time a after the time A. 
   The environment of c may be estimated from the environment of b, or vice versa. For example, when the outside air temperature at the time A is 20° C., the outside air temperature after an elapse of the time a may be increased by a few degrees (e.g. 2° C.) to 22° C. or be decreased by a few degrees (e.g. 2° C.) to 18° C. Conversely, when the outside air temperature after an elapse of the time a from the time A is 20° C., the outside air temperature before an elapse of the time a (at the time A) may be decreased by a few degrees (e.g. 2° C.) to 18° C. or be increased by a few degrees (e.g. 2° C.) to 22° C. Here, whether to increase or decrease the temperature, and how many degrees the temperature is increased or decreased may be controlled depending on time zones and the like. 
   When the fault diagnostic apparatus according to the sixth aspect of the present invention is used, a temperature of the object to be examined after an elapse of a predetermined time period is estimated based on a temperature of the object to be examined detected by the temperature detector at the prescribed time, a parameter at the prescribed time and/or a parameter after an elapse of the predetermined time period from the prescribed time. As a result, the temperature of the object to be examined after an elapse of the predetermined time period can be estimated with good accuracy. 
   A fault diagnostic apparatus according to a seventh aspect of the present invention is characterized by further comprising a correcting unit for correcting an estimated temperature to be estimated by the temperature estimating unit in a manner that makes the estimated temperature closer to a measured temperature to be detected by the temperature detector and a learning unit that learns a correction value by the correcting unit based on a deviation between an estimated temperature of the object to be examined estimated by the temperature estimating unit and a measured temperature detected by the temperature detector in a case where the temperature detector was judged as being normal in any one of the fault diagnostic apparatuses according to the third to sixth aspects of the present invention. 
   When a large difference is caused between an estimated temperature to be estimated by the temperature estimating unit and a measured temperature, it is judged that there is a fault in the temperature detector. Therefore, in the case of low estimate accuracy, there is a possibility that it may be judged that there is a fault in the temperature detector, regardless of no fault, or conversely, there is a possibility that it may be judged that there is no fault in the temperature detector, regardless of an occurrence of a fault. 
   When the fault diagnostic apparatus according to the seventh aspect of the present invention is used, an estimated temperature to be estimated by the temperature estimating unit is corrected in a manner that makes the estimated temperature closer to a measured temperature to be detected by the temperature detector, and the correction value is learned based on a deviation between the estimated temperature and the measured temperature in a case where the temperature detector was judged as being normal. Thus, the accuracy of the estimated temperature of the object to be examined can be improved and the accuracy of the diagnosis can be improved. 
   A fault diagnostic apparatus according to an eighth aspect of the present invention is characterized by the parameter which is at least one of an outside air temperature and an internal combustion engine oil temperature in any one of the fault diagnostic apparatuses according to the third to seventh aspects of the present invention. 
   When the fault diagnostic apparatus according to the eighth aspect of the present invention is used, at least one of an outside air temperature and an oil temperature of an internal combustion engine (such as an engine oil temperature) is used as the parameter. In a situation where the temperature of the internal combustion engine falls, the outside air temperature or the engine oil temperature greatly influences heat radiation of the object to be examined (such as cooling water). For example, when the cooling water is 60° C., its temperature drop rate in the case of an outside air temperature of −10° C. is overwhelmingly higher than that in the case of an outside air temperature of 30° C. 
   Since the outside air temperature and the internal combustion engine oil temperature which greatly influence heat radiation of the object to be examined are used as the parameters, the estimated temperature of the object to be examined can be more properly determined. 
   A fault diagnostic apparatus according to a ninth aspect of the present invention is characterized by an estimated temperature which is corrected based on a parameter related to a temperature drop of the object to be examined in any one of the fault diagnostic apparatuses according to the third to eighth aspects of the present invention. 
   A fault diagnostic apparatus according to a tenth aspect of the present invention is characterized by the parameter which is at least one of an atmospheric pressure and location information of the vehicle in the fault diagnostic apparatus according to the ninth aspect of the present invention. 
   Even under the same condition of outside air temperature, it is considered that a difference is caused in temperature drop of the object to be examined by a difference in an environment other than the outside air temperature. For example, the rate of drop in temperature of cooling water is different between at a sea level altitude of 0 m and above 1000 m, even if the outside air temperature is 4° C. at the both places. 
   When the fault diagnostic apparatus according to the ninth or tenth aspect of the present invention is used, the estimated temperature is corrected based on a parameter related to a temperature drop of the object to be examined, leading to further improvement of the accuracy of the estimated temperature. 
   Here, as the parameter, an atmospheric pressure and location information of the vehicle are exemplified. From the location information of the vehicle, an altitude where the vehicle is located can be recognized, or it can be recognized that the vehicle is in an urban area (where the rate of temperature drop of cooling water becomes lower) or that the vehicle is in a mountainous region where cool wind blows (where the rate of temperature drop of cooling water becomes higher). 
   A fault diagnostic apparatus according to an eleventh aspect of the present invention is characterized by the fault diagnostic device, comprising a temperature estimating unit for estimating a temperature of the object to be examined based on location information of the vehicle and a temperature drop characteristic of the object to be examined in a specified place, and a temperature comparing unit for comparing an estimated temperature estimated by the temperature estimating unit with a temperature detected by the temperature detector, the fault diagnostic device which diagnoses a fault of the temperature detector based on a comparison result by the temperature comparing unit in the fault diagnostic apparatus according to the second aspect of the present invention. 
   A fault diagnostic apparatus according to a twelfth aspect of the present invention is characterized by the specified place which is a parking lot where the vehicle is frequently parked in the fault diagnostic apparatus according to the eleventh aspect of the present invention. 
   It is thought that in a specified place, there is a temperature drop characteristic of the object to be examined in accordance with the place. By making use of this characteristic, the estimate accuracy of temperature can be improved. 
   When the fault diagnostic apparatus according to the eleventh or twelfth aspect of the present invention is used, the temperature of the object to be examined is estimated based on the temperature drop characteristic of the object to be examined in the specified place, the estimated temperature estimated by the temperature estimating unit and a temperature detected by the temperature detector are compared with each other, and based on this comparison result, a fault of the temperature detector is diagnosed. 
   Therefore, when the vehicle is parked in the specified place, the estimated temperature of the object to be examined is determined with good accuracy, based on the temperature drop characteristic of the object to be examined in the specified place. Thus, the accuracy of the fault diagnosis of the temperature detector can also be improved. Here, as the specified place, a parking lot where the vehicle is frequently parked (such as a garage of a user&#39;s house, a parking lot of the user&#39;s office and a parking pool of a supermarket which the user often visits) is exemplified. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram schematically showing the principal part of an engine control unit wherein a fault diagnostic apparatus according to a first embodiment of the present invention is adopted; 
       FIG. 2  is a map of a memory format of a database; 
       FIG. 3  is a flowchart showing a processing operation performed by a microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the first embodiment is adopted; 
       FIG. 4  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the first embodiment is adopted; 
       FIG. 5  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the first embodiment is adopted; 
       FIG. 6  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the first embodiment is adopted; 
       FIG. 7  is an example of a memory format of the database; 
       FIG. 8  is a flowchart showing a processing operation performed by a microcomputer in an engine control unit wherein a fault diagnostic apparatus according to another embodiment is adopted; 
       FIG. 9  is a block diagram schematically showing the principal part of an engine control unit wherein a fault diagnostic apparatus according to a second embodiment is adopted; 
       FIG. 10  is a flowchart showing a processing operation performed by a microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the second embodiment is adopted; 
       FIG. 11  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the second embodiment is adopted; 
       FIG. 12  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the second embodiment is adopted; 
       FIG. 13  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the second embodiment is adopted; 
       FIG. 14  is a block diagram schematically showing the principal part of an engine control unit wherein a fault diagnostic apparatus according to a third embodiment is adopted; 
       FIG. 15  is an example of a memory format of a database; 
       FIG. 16  is a flowchart showing a processing operation performed by a microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the third embodiment is adopted; 
       FIG. 17  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the third embodiment is adopted; 
       FIG. 18  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the third embodiment is adopted; and 
       FIG. 19  is a flowchart showing a processing operation performed by the microcomputer in the engine control unit wherein the fault diagnostic apparatus according to the third embodiment is adopted. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the fault diagnostic apparatus according to the present invention are described below by reference to the Figures noted above. 
     FIG. 1  is a block diagram schematically showing the principal part of an engine control unit wherein a fault diagnostic apparatus according to a first embodiment is adopted. 
   Reference numeral  1  in  FIG. 1  represents an engine control unit, comprising a microcomputer  2  having a CPU, a ROM and a RAM which are not shown and a database  5 , an input circuit  3 , an output circuit  4 , a soak timer  6  for realizing a power supply to the engine control unit  1  at the end of an elapse of a predetermined time T (e.g. 180 minutes) after an IG switch is turned off (i.e. an electric power supply to the engine control unit  1  is shut off), and an EEPROM  7 . Here, the database  5  may be arranged not within the microcomputer  2  but outside the microcomputer  2 , though the rate of processing data becomes lower. 
   The microcomputer  2  captures information through the input circuit  3  from sensors of each kind (e.g. an air volume sensor  11 , a crank angle sensor  12 , a water temperature sensor  13  for detecting a temperature of cooling water for cooling an engine, and a throttle position sensor  14 ) and the like, performs computing processing of each kind, and conducts engine control and the like through the output circuit  4 . In addition, an outside air temperature sensor  15  is connected through the input circuit  3  to the microcomputer  2 , which can grasp an outside air temperature. 
     FIG. 2  is a map showing relationships among a water temperature at a stop of the engine, an outside air temperature at the stop of the engine, an outside air temperature after an elapse of the predetermined time T from the stop of the engine and a water temperature that drops in an elapse of the predetermined time T after the stop of the engine. This is turned into data form and the data is stored in the database  5 . 
   The map shown in  FIG. 2  indicates that a water temperature that drops in an elapse of the predetermined time T after a stop of the engine is ‘d8 23 ’, where a water temperature at the stop of the engine is 80° C. or above, an outside air temperature at that time is between 30° C. and 39° C. and an outside air temperature after an elapse of the predetermined time T from the stop of the engine is between 20° C. and 29° C. For example, in a case where the water temperature at the stop of the engine is 82° C., the outside air temperature at that time is 33° C., the outside air temperature after an elapse of the predetermined time T from the stop of the engine is 28° C. and the drop water temperature d8 23  is set to be ‘20° C.’, the water temperature after an elapse of the predetermined time T from the stop of the engine can be estimated as being 62°C.(=82°C.−20°C.) from the map shown in  FIG. 2 . 
   A processing operation [1−1] performed by the microcomputer  2  in the engine control unit  1  wherein the fault diagnostic apparatus according to the first embodiment is adopted is described below with a flowchart shown in  FIG. 3 . Here, the processing operation [ 1 - 1 ] is conducted every prescribed time period. First, whether the ignition (IG) switch was turned from the on position to the off position or not is judged (Step S 1 ). When it is judged that the IG switch was turned from the on position to the off position (i.e. the engine came to a stop), information about a temperature of cooling water (water temperature WT 1 ) obtained from the water temperature sensor  13  is captured (Step S 2 ), information about a temperature of outside air (outside air temperature OT 1 ) obtained from the outside air temperature sensor  15  is captured (Step S 3 ), and these pieces of temperature information are stored in the EEPROM 7 (Step S 4 ). 
   A processing operation [1-2] performed by the microcomputer  2  in the engine control unit  1  wherein the fault diagnostic apparatus according to the first embodiment is adopted is described below with a flowchart shown in  FIG. 4 . Here, the processing operation [1-2] is conducted when electric power was provided by the soak timer  6  (i.e. when the predetermined time T elapsed after the stop of the engine). In other words, the processing operation [1-2] is conducted in cases where the engine has been at a stop for the predetermined time T. 
   Information about a temperature of cooling water (water temperature WT 2 ) obtained from the water temperature sensor  13  is captured (Step S 11 ), information about a temperature of outside air (outside air temperature OT 2 ) obtained from the outside air temperature sensor  15  is captured (Step S 12 ), and a water temperature WT 2 ′ of the cooling water after an elapse of the predetermined time T from the stop of the engine is estimated (Step S 13 ). A method of estimating the water temperature WT 2 ′ is described in detail below with a flowchart shown in  FIG. 5 . 
   After estimating the water temperature WT 2 ′, whether or not a difference between the water temperature WT 2  detected by the water temperature sensor  13  and the estimated water temperature WT 2 ′ is not less than a prescribed value Δ WT is judged (Step S 14 ). When it is judged that the difference between the water temperature WT 2  and the estimated water temperature WT 2 ′ is not less than the prescribed value Δ WT (i.e. the water temperature WT 2  detected by the water temperature sensor  13  is far different from the estimated water temperature WT 2 ′), it is decided that there is a fault in the water temperature sensor  13  (Step S 15 ). On the other hand, when it is judged that the difference between the water temperature WT 2  and the estimated water temperature WT 2 ′ is less than the prescribed value Δ WT, it is decided that the water temperature sensor  13  is in a normal condition (Step S 16 ). 
   A processing operation [1-3] performed by the microcomputer  2  in the engine control unit  1  wherein the fault diagnostic apparatus according to the first embodiment is adopted is described below with a flowchart shown in  FIG. 5 . Here, the processing operation [1-3] is conducted in Step S 13  “Calculate Estimated Water Temperature WT 2 ′” shown in  FIG. 4 . 
   Information showing the water temperature WT 1  and the outside air temperature OT 1  at the stop of the engine is read from the EEPROM 7 (Steps S 21  and S 22 ). And based on the water temperature WT 1  at the stop of the engine, the outside air temperature OT 1  at the stop of the engine and the outside air temperature OT 2  after an elapse of the predetermined time T from the stop of the engine, a water temperature dn ij  (e.g. d8 11 ) that drops in an elapse of the predetermined time T after the stop of the engine is obtained from the map information (see  FIG. 2 ) stored in the database  5  (Step S 23 ). Thereafter, by subtracting the drop water temperature dn ij  from the water temperature WT 1 , the estimated water temperature WT 2 ′ is calculated (Step S 24 ). 
   Here, the outside air temperature is adopted as a parameter related to a quantity of radiant heat of the cooling water (however, when the outside air temperature is higher than the temperature of the cooling water, the outside air temperature becomes a parameter related to a heating value to the cooling water), and the temperature of the cooling water is estimated. But the parameter related to the quantity of radiant heat of the cooling water or the heating value to the cooling water is not limited to the outside air temperature. In another embodiment, for example, as a parameter related to a quantity of radiant heat of cooling water or a heating value to the cooling water, an engine oil temperature may be adopted. It is needless to say that a plurality of parameters may be adopted. 
   The processing operations [1-1] to [1-3] which are characteristics of the apparatus according to the present invention, performed by the microcomputer  2  in the engine control unit  1  wherein the fault diagnostic apparatus according to the first embodiment is adopted were separately described above. An overall processing operation [1-T] performed by the microcomputer  2  is described below with a flowchart shown in  FIG. 6 . Here, the processing operation [1-T] is performed when the microcomputer  2  started. 
   First, initial processing of the microcomputer  2  is conducted (Step S 101 ). Then, whether the IG switch is in the on position or not is judged (Step S 102 ). Here, as the initial processing, processing for transferring data in a stand-by RAM to a normal RAM is exemplified. 
   When it is judged that the IG switch is in the on position (i.e. the microcomputer  2  was caused to start by an IG key operation), whether there is a fault in the sensors (such as the water temperature sensor  13 ), an actuator and a wire harness of an engine control system or not is judged (Step S 103 ). In other words, whether or not it was decided that there was a fault through the below-described fault diagnostic processing in Step S 109 , or whether or not it was decided that there was a fault in the water temperature sensor  13  in Step S 15  is judged. 
   When it is judged that there is a fault in the sensor or the like, a fault warning lamp is lit up (Step S 104 ), fail-safe processing is performed (Step S 105 ), and then, the operation goes to Step S 106 . As the fail-safe processing, processing for setting abnormal data to be a fixed value is exemplified. For example, when there is a fault in the water temperature sensor  13 , water temperature data is fixed at 80° C. 
   On the other hand, when it is judged that there is no fault in the sensor or the like in Step S 103 , the operation directly goes to Step S 106  since the processing in Steps S 104  and S 105  is not required. 
   Data of each kind is input (Step S 106 ), computing processing is performed (Step S 107 ), engine control is conducted by outputting data obtained through the computation (Step S 108 ), fault diagnostic processing is performed on the sensors and the like of the engine control system (Step S 109 ), and then, the operation returns to Step S 102 . As the engine control, fuel injection control, ignition timing control, knock control and idle speed control are exemplified. 
   When it is judged that the IG switch is not in the on position in Step S 102 , whether the IG switch was turned from the on position to the off position or not is judged (Step S 110 ). When it is judged that the IG switch was turned from the on position to the off position (i.e. the engine came to a stop), the processing operation [1-1] (Steps S 1 -S 4 ) shown in  FIG. 3  is conducted. 
   On the other hand, when it is judged that the IG switch was not turned from the on position to the off position (i.e. the IG switch has been in the off position), this starting of the microcomputer  2  is a start caused by the soak timer  6  while the engine is at a stop. Then, the processing operation [1-2] (Steps S 11 -S 16 ) shown in  FIG. 4  is conducted. 
   By using the fault diagnostic apparatus according to the first embodiment, in a situation where the engine temperature to be a chief source of heating the cooling water decreases, a fault of the water temperature sensor  13  is diagnosed. In the situation where the engine temperature decreases, the temperature of the cooling water also falls gradually with the decrease. Therefore, by monitoring a change in temperature of the cooling water, a fault of the water temperature sensor  13  can be diagnosed. Moreover, since the engine temperature does not rise, it becomes easier to estimate a change in temperature of the cooling water. Thus, the accuracy of the fault diagnosis of the water temperature sensor  13  can be improved. 
   As described above, when a difference of the prescribed value Δ WT or above is caused between the estimated water temperature WT 2 ′ after an elapse of the predetermined time T from the stop of the engine and the measured water temperature WT 2  detected by the water temperature sensor  13 , it is judged that there is a fault in the water temperature sensor  13 . Therefore, in the case of low estimate accuracy, there is a possibility that it may be judged that there is a fault in the water temperature sensor  13 , regardless of no fault, or conversely, there is a possibility that it may be judged that there is no fault in the water temperature sensor  13 , regardless of an occurrence of a fault. 
   In another embodiment, by using a map showing relationships among a water temperature WT 1  at a stop of the engine, an outside air temperature OT 1  at the stop of the engine, an outside air temperature OT 2  after an elapse of a predetermined time T from the stop of the engine, a water temperature dn ij  that drops in an elapse of the predetermined time T after the stop of the engine and a correction value Adn ij  for correcting the drop water temperature dn ij  as shown in  FIG. 7 , the drop water temperature dn ij  may be corrected and the estimated water temperature WT 2 ′ (=WT 1 −dn ij +Adn ij ) may be made closer to a measured water temperature WT 2  to be detected by the water temperature sensor  13 . Here, the correction value Adn ij  is learned based on a deviation Δ WT 2  between the measured water temperature WT 2  detected by the water temperature sensor  13  in a case where the sensor was judged as being normal and the estimated water temperature WT 2 ′, and its initial value is ‘0° C.’. And this map may be turned into data form and be stored in the database  5 . 
   The map shown in  FIG. 7  tells that a water temperature which drops in an elapse of the predetermined time T after the stop of the engine is ‘d8 23 ’, where a water temperature at the stop of the engine is 80° C. or above, an outside air temperature at that time is between 30° C. and 39° C. and an outside air temperature after an elapse of the predetermined time T from the stop of the engine is between 20° C. and 29° C., and that the correction value at that time is ‘Ad8 23 ’. 
   For example, in a case where the water temperature at the stop of the engine is 82° C., the outside air temperature at that time is 33° C., the outside air temperature after an elapse of the predetermined time T from the stop of the engine is 28° C., the drop water temperature d8 23  is set to be ‘20° C.’ and the correction value Ad8 23  is set to be ‘2° C.’, the water temperature after an elapse of the predetermined time T from the stop of the engine can be estimated as being 64° C. (=82° C.−20° C.+2° C.) from the map shown in  FIG. 7 . 
   Learning of the correction value Adn ij  is described below with a flowchart shown in  FIG. 8 . Here, this processing operation is conducted after the water temperature sensor  13  was judged as being in a normal condition in Step S 16  of  FIG. 4 . First, the deviation Δ WT 2  between the measured water temperature WT 2  and the estimated water temperature WT 2 ′ is found (Step S 111 ), and whether the deviation Δ WT 2  is larger than an upper limit of LA (&gt;0) or not is judged (Step S 112 ). When it is judged that the deviation Δ WT 2  is larger than the upper limit of LA, the deviation Δ WT 2  is set to be the upper limit of LA (Step S 113 ), and then, the operation goes to Step S 116 . 
   On the other hand, when it is judged that the deviation Δ WT 2  is not larger than the upper limit of LA, whether the deviation Δ WT 2  is smaller than a lower limit of LB (&lt;0) or not is judged (Step S 114 ). When it is judged that the deviation Δ WT 2  is smaller than the lower limit of LB, the deviation Δ WT 2  is set to be the lower limit of LB (Step S 115 ), and then, the operation goes to Step S 116 . When it is judged that the deviation Δ WT 2  is not smaller than the lower limit of LB, the operation directly goes to Step S 116 . 
   The correction value Adn ij  (e.g. Ad8 11 ) is read from the map information (see  FIG. 7 ) stored in the database  5  based on the water temperature WT 1  at the stop of the engine, the outside air temperature OT 1  at the stop of the engine and the outside air temperature OT 2  after an elapse of the predetermined time T from the stop of the engine (Step S 116 ). And then, leveling processing on the correction value Adn ij  is performed by using the below-mentioned mathematical expression (Step S 117 ).
 
 Adn   ij ′←{( N−Y ) Adn   ij   +Y·ΔWT 2 }/N  
 
   By comparing this time correction value Adn ij ′ with the preceding correction value Adn ij , whether the correction value Adn ij ′ has changed in a direction receding from 0 (receding direction) or not is judged (Step S 118 ). When it is judged that the correction value Adn ij ′ has changed in the receding direction, ‘1’ is added to a receding-side counter CAn ij  (Step S 119 ) and a convergent-side counter CBn ij  is set at ‘0’ (Step S 120 ). 
   Whether the receding-side counter CAn ij  indicates a prescribed number of times of CA or more is judged (Step S 121 ). When it is judged that the receding-side counter CAn ij  indicates the prescribed number of times of CA or more (i.e. the correction value Adn ij  has receded from 0 in succession), the correction value Adn ij ′ is multiplied by a given value α (a numerical value larger than 1 such as 1.2) (Step S 122 ). And then, the correction value Adn ij  in the database  5  is updated to the correction value Adn ij ′ (Step S 123 ). 
   On the other hand, when it is judged that the receding-side counter CAn ij  does not indicate the prescribed number of times of CA or more, Step S 122  is bypassed and the operation goes to Step S 123 , wherein the correction value Adn ij  in the database  5  is updated to the correction value Adn ij ′. 
   When it is judged that the correction value Adn ij ′ has not changed in the receding direction in Step S 118 , by comparing the this time correction value Adn ij ′ with the preceding correction value Adn ij , whether the correction value Adn ij ′ has changed in a direction approaching 0 (convergent direction) or not is judged (Step S 124 ). When it is judged that the correction value Adn ij ′ has changed in the convergent direction, ‘1’ is added to the convergent-side counter CBn ij  (Step S 125 ) and the receding-side counter CAn ij  is set at ‘0’ (Step S 126 ). 
   Whether the convergent-side counter CBn ij  indicates a prescribed number of times of CB or more is judged (Step S 127 ). When it is judged that the convergent-side counter CBn ij  indicates the prescribed number of times of CB or more (i.e. the correction value Adn ij  has approached 0 in succession), the correction value Adn ij ′ is multiplied by a given value β (a numerical value smaller than 1 such as 0.8) (Step S 128 ). And then, the correction value Adn ij  in the database  5  is updated to the correction value Adn ij ′ (Step S 123 ). 
   On the other hand, when it is judged that the convergent-side counter CBn ij  does not indicate the prescribed number of times of CB or more, Step S 128  is bypassed and the operation goes to Step S 123 , wherein the correction value Adn ij  in the database  5  is updated to the correction value Ad nj ′. 
   When it is judged that the correction value Adn ij ′ has not changed in the convergent direction (i.e. the correction value Adn ij ′ has changed neither in the receding direction nor in the convergent direction), this processing operation is concluded at once since it is not necessary to update the correction value Adn ij  in the database  5 . 
     FIG. 9  is a block diagram schematically showing the principal part of an engine control unit wherein a fault diagnostic apparatus according to a second embodiment is adopted. Here, the same components as those of the engine control unit  1  shown in  FIG. 1  are similarly marked. 
   Reference numeral  21  in  FIG. 9  represents an engine control unit, comprising a microcomputer  22  having a CPU, a ROM and a RAM which are not shown and a database  5 , an input circuit  3 , an output circuit  4 , a soak timer  6  for realizing a power supply to the engine control unit  21  at the end of an elapse of a predetermined time T after an IG switch is turned off (i.e. an electric power supply to the engine control unit  21  is shut off), and an EEPROM  7 . Here, the database  5  may be arranged not within the microcomputer  22  but outside the microcomputer  22 , though the rate of processing data becomes lower. 
   The microcomputer  22  captures information through the input circuit  3  from sensors of each kind (e.g. an air volume sensor  11 , a crank angle sensor  12 , a water temperature sensor  13  for detecting a temperature of cooling water for cooling an engine, and a throttle position sensor  14 ) and the like, performs computing processing of each kind, and conducts engine control and the like through the output circuit  4 . In addition, an outside air temperature sensor  15  and an atmospheric pressure sensor  23  are connected through the input circuit  3  to the microcomputer  22 , which can grasp an outside air temperature and an atmospheric pressure. 
   A processing operation [2-1] performed by the microcomputer  22  in the engine control unit  21  wherein the fault diagnostic apparatus according to the second embodiment is adopted is described below with a flowchart shown in  FIG. 10 . Here, the processing operation [2-1] is conducted every prescribed time period. First, whether the IG switch was turned from the on position to the off position or not is judged (Step S 31 ). When it is judged that the IG switch was turned from the on position to the off position (i.e. the engine came to a stop), information about a temperature of cooling water (water temperature WT 1 ) obtained from the water temperature sensor  13  is captured (Step S 32 ), and information about a temperature of outside air (outside air temperature OT) obtained from the outside air temperature sensor  15  is captured (Step S 33 ). 
   Furthermore, information about an atmospheric pressure (atmospheric pressure AP) obtained from the atmospheric pressure sensor  23  is captured (Step S 34 ). And the water temperature information, the outside air temperature information and the atmospheric pressure information are stored in the EEPROM  7  (Step S 35 ). 
   A processing operation [2-2] performed by the microcomputer  22  in the engine control unit  21  wherein the fault diagnostic apparatus according to the second embodiment is adopted is described below with a flowchart shown in  FIG. 11 . Here, the processing operation [2-2] is conducted when electric power was provided by the soak timer  6  (i.e. when the predetermined time T elapsed after the stop of the engine). In other words, the processing operation [2-2] is conducted in cases where the engine has been at a stop for the predetermined time T. 
   Information about a temperature of cooling water (water temperature WT 2 ) obtained from the water temperature sensor  13  is captured (Step S 41 ), information about a temperature of outside air (outside air temperature OT 2 ) obtained from the outside air temperature sensor  15  is captured (Step S 42 ), and a water temperature WT 2 ′ of the cooling water after an elapse of the predetermined time T from the stop of the engine is estimated (Step S 43 ). A method of estimating the water temperature WT 2 ′ is described in detail below with a flowchart shown in  FIG. 12 . 
   After estimating the water temperature WT 2 ′, whether or not a difference between the water temperature WT 2  detected by the water temperature sensor  13  and the estimated water temperature WT 2 ′ is not less than a prescribed value Δ WT is judged (Step S 44 ). When it is judged that the difference between the water temperature WT 2  and the estimated water temperature WT 2 ′ is not less than the prescribed value Δ WT (i.e. the water temperature WT 2  detected by the water temperature sensor  13  is far different from the estimated water temperature WT 2 ′), it is decided that there is a fault in the water temperature sensor  13  (Step S 45 ). On the other hand, when it is judged that the difference between the water temperature WT 2  and the estimated water temperature WT 2 ′ is less than the prescribed value Δ WT, it is decided that the water temperature sensor  13  is in a normal condition (Step S 46 ). 
   A processing operation [2-3] performed by the microcomputer  22  in the engine control unit  21  wherein the fault diagnostic apparatus according to the second embodiment is adopted is described below with a flowchart shown in  FIG. 12 . Here, the processing operation [2-3] is conducted in Step S 43  “Calculate Estimated Water Temperature WT 2 ′” shown in  FIG. 11 . 
   Information showing the water temperature WT 1  and the outside air temperature OT 1  at the stop of the engine is read from the EEPROM  7  (Steps S 51  and S 52 ). And based on the water temperature WT 1  at the stop of the engine, the outside air temperature OT  1  at the stop of the engine and the outside air temperature OT 2  after an elapse of the predetermined time T from the stop of the engine, a water temperature dn ij  (e.g. d8 11 ) that drops in an elapse of the predetermined time T after the stop of the engine is obtained from the map information (see  FIG. 2 ) stored in the database  5  (Step S 53 ). Thereafter, by subtracting the drop water temperature dn ij  from the water temperature WT 1 , the estimated water temperature WT 2 ′ is calculated (Step S 54 ). 
   Then, information showing the atmospheric pressure AP (an atmospheric pressure at a place where a user&#39;s vehicle is at a stop) is read from the EEPROM  7  (Step S 55 ). Whether or not the atmospheric pressure AP is not more than a prescribed atmospheric pressure AP′ is judged (Step S 56 ). When it is judged that the atmospheric pressure AP is not more than the prescribed atmospheric pressure AP′ (i.e. the atmospheric pressure is low and the rate of temperature drop becomes higher than usual), the estimated water temperature WT 2 ′ is determined by multiplying the estimated water temperature WT 2 ′ by a factor K (&gt;1) (Step S 57 ). On the other hand, when it is judged that the atmospheric pressure AP is more than the prescribed atmospheric pressure AP′, the processing operation [2-3] is concluded at once. 
   The processing operations [2-1] to [2-3] which are characteristics of the apparatus according to the present invention, performed by the microcomputer  22  in the engine control unit  21  wherein the fault diagnostic apparatus according to the second embodiment is adopted were separately described above. An overall processing operation [2-T] performed by the microcomputer  22  is as shown in  FIG. 13 . 
   By using the fault diagnostic apparatus according to the second embodiment, since the temperature of the cooling water is estimated in consideration of the atmospheric pressure AP as a parameter related to a drop in temperature of the cooling water, the estimate accuracy can be improved. Here, in cases where the atmospheric pressure AP is not more than the prescribed atmospheric pressure AP′, the estimated water temperature is corrected simply by multiplying the estimated water temperature WT 2 ′ by the factor K. But the method of correction is not limited to this method. In another embodiment, the factor K may be set according to how high the atmospheric pressure AP is. 
   For example, when the atmospheric pressure AP is not less than an atmospheric pressure AP 1 , the factor K is set to be less than 1. When the atmospheric pressure AP is not less than an atmospheric pressure AP 2  (AP 2 &lt;AP 1 ) and less than the atmospheric pressure AP 1 , the factor K is set to be 1. And when the atmospheric pressure AP is less than the atmospheric pressure AP 2 , the factor K is set to be more than 1. 
     FIG. 14  is a block diagram schematically showing the principal part of an engine control unit wherein a fault diagnostic apparatus according to a third embodiment is adopted. Here, the same components as those of the engine control unit  1  shown in  FIG. 1  are similarly marked. 
   Reference numeral  31  in  FIG. 14  represents an engine control unit, comprising a microcomputer  32  having a CPU, a ROM and a RAM, an input circuit  3 , an output circuit  4 , a database  35 , a soak timer  6  for realizing a power supply to the engine control unit  31  at the end of an elapse of a predetermined time T after an IG switch is turned off (i.e. an electric power supply to the engine control unit  31  is shut off), and an EEPROM  7 . 
   The microcomputer  32  captures information through the input circuit  3  from sensors of each kind (e.g. an air volume sensor  11 , a crank angle sensor  12 , a water temperature sensor  13  for detecting a temperature of cooling water for cooling an engine, and a throttle position sensor  14 ) and the like, performs computing processing of each kind, and can conduct engine control and the like through the output circuit  4 . In addition, an outside air temperature sensor  15  and an atmospheric pressure sensor  23  are connected through the input circuit  3  to the microcomputer  32 , which can grasp an outside air temperature and an atmospheric pressure. 
   Moreover, a navigation system  33  is connected to the microcomputer  32 , which can receive information showing that a current position of a user&#39;s vehicle is in an urban area, or in a mountainous region, or in a garage of the user&#39;s house sent from the navigation system  33  as navigation information. 
     FIG. 15  is a map showing relationships among a water temperature at a stop of the engine in a specified place (here, a garage of a user&#39;s house), an outside air temperature at the stop of the engine, an outside air temperature after an elapse of a predetermined time T from the stop of the engine and a water temperature that drops in an elapse of the predetermined time T after the stop of the engine. This is turned into data form and the data is stored in the database  35 . In the database  35 , not only the data made of the map (special map) shown in  FIG. 15  but also the data made of the map (general map) shown in  FIG. 2  are stored. Here, the database  35  may be arranged within the microcomputer  32  (such as the CPU). As a result, the rate of processing becomes higher, resulting in power savings. 
   A processing operation [3-1] performed by the microcomputer  32  in the engine control unit  31  wherein the fault diagnostic apparatus according to the third embodiment is adopted is described below with a flowchart shown in  FIG. 16 . Here, the processing operation [3-1] is conducted every prescribed time period. First, whether the IG switch was turned from the on position to the off position or not is judged (Step S 61 ). When it is judged that the IG switch was turned from the on position to the off position (i.e. the engine came to a stop), information about a temperature of cooling water (water temperature WT 1 ) obtained from the water temperature sensor  13  is captured (Step S 62 ), and information about a temperature of outside air (outside air temperature OT 1 ) obtained from the outside air temperature sensor  15  is captured (Step S 63 ). 
   Furthermore, information about an atmospheric pressure (atmospheric pressure AP) obtained from the atmospheric pressure sensor  23  is captured (Step S 64 ). Then, the microcomputer  32  requests that the navigation system  33  sends navigation information and captures the navigation information sent from the navigation system  33  (Step S 65 ). The water temperature information, the outside air temperature information, the atmospheric pressure information and the navigation information are stored in the EEPROM  7  (Step S 66 ). 
   A processing operation [3-2] performed by the microcomputer  32  in the engine control unit  31  wherein the fault diagnostic apparatus according to the third embodiment is adopted is described below with a flowchart shown in  FIG. 17 . Here, the processing operation [3-2] is conducted when electric power was provided by the soak timer  6  (i.e. when the predetermined time T elapsed after the stop of the engine). In other words, the processing operation [3-2] is conducted in cases where the engine has been at a stop for the prescribed time T. 
   Information about a temperature of cooling water (water temperature WT 2 ) obtained from the water temperature sensor  13  is captured (Step S 71 ), information about a temperature of outside air (outside air temperature OT 2 ) obtained from the outside air temperature sensor  15  is captured (Step S 72 ), and a water temperature WT 2 ′ of the cooling water after an elapse of the predetermined time T from the stop of the engine is estimated (Step S 73 ). A method of estimating the water temperature WT 2 ′ is described in detail below with a flowchart shown in  FIG. 18 . 
   After estimating the water temperature WT 2 ′, whether or not a difference between the water temperature WT 2  detected by the water temperature sensor  13  and the estimated water temperature WT 2 ′ is not less than a prescribed value Δ WT is judged (Step S 74 ). When it is judged that the difference between the water temperature WT 2  and the estimated water temperature WT 2 ′ is not less than the prescribed value Δ WT (i.e. the water temperature WT 2  detected by the water temperature sensor  13  is far different from the estimated water temperature WT 2 ′), it is decided that there is a fault in the water temperature sensor  13  (Step S 75 ). On the other hand, when it is judged that the difference between the water temperature WT 2  and the estimated water temperature WT 2 ′ is less than the prescribed value Δ WT, it is decided that the water temperature sensor  13  is in a normal condition (Step S 76 ). 
   A processing operation [3-3] performed by the microcomputer  32  in the engine control unit  31  wherein the fault diagnostic apparatus according to the third embodiment is adopted is described below with a flowchart shown in  FIG. 18 . Here, the processing operation [3-3] is conducted in Step S 73  “Calculate Estimated Water Temperature WT 2 ′” shown in  FIG. 17 . 
   Information showing the water temperature WT 1  and the outside air temperature OT 1  at the stop of the engine and navigation information are read from the EEPROM  7  (Steps S 81 -S 83 ). And based on the read-out navigation information, whether the user&#39;s vehicle is in a specified place (here, a garage of the user&#39;s house) or not is judged (Step S 84 ). 
   When it is judged that the user&#39;s vehicle is not in the garage of the user&#39;s house, based on the water temperature WT  1  at the stop of the engine, the outside air temperature OT 1  at the stop of the engine and the outside air temperature OT 2  after an elapse of the predetermined time T from the stop of the engine, a water temperature dn ij  (e.g. d8 11 ) that drops in an elapse of the predetermined time T after the stop of the engine is obtained from the general map information (see  FIG. 2 ) stored in the database  35  (Step S 85 ). Thereafter, by subtracting the drop water temperature dn ij  from the water temperature WT 1 , the estimated water temperature WT 2 ′ is calculated (Step S 86 ). 
   Based on the navigation information, whether the user&#39;s vehicle is in an urban area or not is judged (Step S 87 ). When it is judged that the user&#39;s vehicle is in an urban area (i.e. the user&#39;s vehicle is in a place where the temperature does not easily decrease), a factor K 1  used for correcting the estimated water temperature WT 2 ′ is set to be a (&gt;1) (Step S 88 ) and the operation goes to Step S 92 . 
   On the other hand, when it is judged that the user&#39;s vehicle is not in an urban area, whether the user&#39;s vehicle is in a mountainous region or not is judged based on the navigation information (Step S 89 ). When it is judged that the user&#39;s vehicle is in a mountainous region (i.e. the user&#39;s vehicle is in a place where the temperature easily decreases), the factor K, used for correcting the estimated water temperature WT 2 ′ is set to be β (&lt;1) (Step S 90 ) and the operation goes to Step S 92 . On the other hand, when it is judged that the user&#39;s vehicle is not in a mountainous region, the factor K 1  is set to be 1 (Step S 91 ) and the operation goes to Step S 92 . 
   Then, information showing the atmospheric pressure AP (an atmospheric pressure at a place where the user&#39;s vehicle is at a stop) is read from the EEPROM  7  (Step S 92 ). Whether or not the atmospheric pressure AP is not more than a prescribed atmospheric pressure AP′ is judged (Step S 93 ). When it is judged that the atmospheric pressure AP is not more than the prescribed atmospheric pressure AP′ (i.e. the atmospheric pressure is low and the rate of temperature drop becomes higher than usual), the estimated water temperature WT 2 ′ is determined by multiplying the estimated water temperature WT 2 ′ by the factor K 1  and a factor K (&gt;1) (Step S 94 ). On the other hand, when it is judged that the atmospheric pressure AP is more than the prescribed atmospheric pressure AP′, the estimated water temperature WT 2 ′ is determined by multiplying the estimated water temperature WT 2 ′ by the factor K 1  (Step S 95 ). 
   When it is judged that the user&#39;s vehicle is in the garage of the user&#39;s house in Step S 84 , based on the water temperature WT 1  at the stop of the engine, the outside air temperature OT 1  at the stop of the engine and the outside air temperature OT 2  after an elapse of the predetermined time T from the stop of the engine, a water temperature Dn ij  (e.g. d8 11 ) that drops in an elapse of the predetermined time T after the stop of the engine is obtained from the special map information (see  FIG. 15 ) stored in the database  35  (Step S 96 ). Thereafter, by subtracting the drop water temperature Dn ij  from the water temperature WT 1 , the estimated water temperature WT 2 ′ is calculated (Step S 97 ). 
   Here, the estimated water temperature WT 2 ′ is calculated by using the map information specified for the case where the user&#39;s vehicle is in the garage of the user&#39;s house, but the specified place is not limited to the garage of the user&#39;s house. A parking lot of the user&#39;s office or a parking pool of a supermarket that the user frequently visits may be adoptable. In another embodiment, map information about these parking places may be previously prepared and when the user&#39;s vehicle is in any one of these parking places, the estimated water temperature WT 2 ′ may be calculated by using the map information corresponding to each place. 
   The processing operations [3-1] to [3-3] which are characteristics of the apparatus according to the present invention, performed by the microcomputer  32  in the engine control unit  31  wherein the fault diagnostic apparatus according to the third embodiment is adopted were separately described above. An overall processing operation [3-T] performed by the microcomputer  32  is as shown in  FIG. 19 . 
   In the fault diagnostic apparatus according to the third embodiment, the water temperature of the cooling water is estimated in consideration of the atmospheric pressure AP and where the user&#39;s vehicle is placed (here, an urban area/a mountainous region) as parameters related to a drop in temperature of the cooling water, leading to improvement of the estimate accuracy. 
   In addition, when the user&#39;s vehicle is in a specified place (the garage of the user&#39;s house), the water temperature of the cooling water is estimated based on a temperature drop characteristic of the cooling water in that place. As a result, the estimate accuracy of temperature of the cooling water can be further improved when the user&#39;s vehicle is in the specified place. Here, the special map shown in  FIG. 15  can be captured by accumulating data and learning. 
   In the first to third embodiments, the cases where the fault diagnostic apparatus is adopted in the engine control unit were described. But, the fault diagnostic apparatus and the engine control unit may be separately arranged. In another embodiment, a fault diagnostic apparatus may be able to receive necessary information from an engine control unit and the result of a diagnosis may be sent from the fault diagnostic apparatus to the engine control unit. 
   As for the fault diagnostic apparatuses according to the first to third embodiments, the cases where a fault of the water temperature sensor  13  for detecting a temperature of cooling water is diagnosed were described. But, a sensor to be a subject of the diagnosis is not limited to the water temperature sensor  13 . In another embodiment, a sensor for detecting a temperature of engine oil or the like may be a subject of a diagnosis. 
   Moreover, the fault diagnostic apparatus according to the present invention is effective not only in checking a sensor for detecting a temperature of cooling water for cooling an engine or a sensor for detecting a temperature of engine oil, but also in a fault diagnosis performed in a situation where a parameter to be a chief source of heating an object to be examined decreases. 
   For example, it is effective in checking a temperature sensor for measuring a water temperature of a bath having an automatic reheat function (i.e. a function of keeping water temperature of a bath at a fixed temperature or above). The automatic reheat function is to reheat the bath water when the water temperature of the bath decreased to a fixed temperature or below. Therefore, if the temperature sensor for measuring the water temperature has a fault, the function does not work. 
   When a temperature detected by the temperature sensor has not decreased after a given time elapsed, regardless of after water in the bath was heated and the heat function stopped, that is, being in a situation where the water temperature falls, the temperature sensor can be judged as being abnormal. 
   And it is effective in checking a temperature sensor mounted on a small kitchen range (e.g. a gas range) having a function of preventing heating of an empty pot or a temperature control function of oil for deep-frying. When a temperature detected by the temperature sensor has not decreased after a given time elapsed, regardless of after the kitchen range was switched off, that is, being in a situation where the temperature falls, the temperature sensor can be judged as being abnormal.