Oil temperature sensor failure detection device and oil temperature sensor failure detection method

Abstract

To provide a technology capable of further accurately suppressing erroneous judgement that an oil temperature sensor is in a failure state.SOLUTION: A failure detection device of an oil temperature sensor measuring the temperature of a hydraulic oil of an automatic transmission mounted on a vehicle, includes an obtaining portion that obtains oil temperature measured by the oil temperature sensor, a first channel in which a refrigerant cooling an engine of a vehicle circulates via the automatic transmission, a deciding portion that decides a premise condition and a failure condition according to a channel of a refrigerant channel switched with a second channel in which a refrigerant circulates without passing through the automatic transmission, and a determining portion that determines whether the failure condition is satisfied or not when the premise condition is satisfied, and determines that the oil temperature sensor is in a failure state when the failure condition is satisfied. The failure condition includes the matter that oil temperature is less than a threshold temperature. The deciding portion decides the premise condition in the case of the first channel and the premise condition in the case of the second channel to be different conditions, and decides the threshold temperature in the case of the second channel to be lower temperature than the case of the first channel.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an apparatus and method for detecting a fault in an oil temperature sensor. [Background technology]

[0002] There is known a technique for detecting a malfunction of an oil temperature sensor that measures the temperature of hydraulic oil in an automatic transmission of a vehicle. Patent Document 1 discloses a technique for preventing an erroneous determination that the oil temperature sensor has malfunctioned. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-85150 Summary of the Invention [Problem to be solved by the invention]

[0004] There was a need for technology that could more accurately prevent the incorrect determination that the oil temperature sensor was faulty. [Means for solving the problem]

[0005] The present disclosure has been made to solve the above-mentioned problems, and can be realized in the following forms.

[0006] (1) According to one aspect of the present disclosure, there is provided a fault detection device for an oil temperature sensor that measures the temperature of hydraulic oil in an automatic transmission mounted on a vehicle. The fault detection device includes: an acquisition unit that acquires the oil temperature measured by the oil temperature sensor; a determination unit that determines a precondition for determining whether to acquire the oil temperature and a fault condition for determining a fault in the oil temperature sensor, based on a refrigerant flow path that switches between a first flow path through which a refrigerant for cooling an engine of the vehicle flows via the automatic transmission and a second flow path through which the refrigerant flows without passing through the automatic transmission; and a determination unit that determines whether the fault condition is satisfied when the precondition is satisfied and determines that the oil temperature sensor has a fault when the fault condition is satisfied. The fault condition includes that the oil temperature is below a threshold temperature. The determination unit determines the precondition when the flow path is the first flow path to be different from the precondition when the flow path is the second flow path, and determines the threshold temperature when the flow path is the second flow path to be lower than the threshold temperature when the flow path is the first flow path. According to this aspect of the fault detection device, the prerequisite and fault condition are determined according to the flow path of the refrigerant flow path. The determination unit determines the threshold temperature for the fault condition in the case of the second flow path to be lower than the threshold temperature for the fault condition in the case of the first flow path. Therefore, even if the refrigerant flow path is the second flow path, the amount of heat received by the automatic transmission is small, and the oil temperature does not rise sufficiently, it is possible to prevent erroneous determination that the oil temperature sensor is faulty. (2) In the fault detection device of the above form, the precondition includes that the cumulative time during which a difference occurs between the engine rotation speed and the input rotation speed of the automatic transmission since the engine is started is equal to or longer than a predetermined first threshold time, and the determination unit may determine the first threshold time when the flow path is the first flow path to be shorter than the first threshold time when the flow path is the second flow path. According to this type of fault detection device, the preconditions include the cumulative time during which a difference between the engine speed and the input speed of the automatic transmission is equal to or greater than a threshold time, so fault conditions can be determined after the oil temperature has risen sufficiently, thereby preventing erroneous determinations. (3) In the failure detection device of the above form, when the flow path is the first flow path, the determination unit may determine the first threshold time to be 0, and determine the preconditions to include one or more of: the elapsed time since the engine was started is equal to or greater than a predetermined second threshold time; the cumulative time that the vehicle has been in an accelerator-on state since the engine was started is equal to or greater than a predetermined third threshold time; and the cumulative mileage of the vehicle since the engine was started is equal to or greater than a predetermined threshold distance. For example, when a vehicle is driven in a normal temperature range that is not low, there may be no difference between the engine speed and the input speed of the automatic transmission. According to this type of failure detection device, even when the flow path is the first flow path and no difference between the engine speed and the input speed of the automatic transmission occurs, it is possible to suppress a decrease in the frequency of determining whether or not the failure condition is met due to the prerequisite not being met, and to suppress an erroneous determination that the oil temperature sensor is faulty.

[0007] The present disclosure can be realized in various forms, for example, in the form of a method for determining prerequisites and fault conditions. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing an overview of an engine system. [Figure 2] FIG. 1 is a schematic diagram illustrating an overview of a failure detection device. [Figure 3] 10 is a flowchart illustrating an example of a failure detection process. DETAILED DESCRIPTION OF THE INVENTION

[0009] A. First embodiment: 1 is a schematic diagram showing an overview of an engine system 110 according to this embodiment. The engine system 110 is a system that is mounted on a vehicle 100 and used as a power source. The engine system 110 includes an engine 10, a refrigerant flow path 20, an automatic transmission 30, an oil temperature sensor 31, and a fault detection device 40.

[0010] The engine 10 is an internal combustion engine that generates power by burning fuel such as gasoline or diesel. The power of the engine 10 is transmitted to an automatic transmission 30.

[0011] The refrigerant flow path 20 is a flow path through which the refrigerant that cools the engine 10 flows. The refrigerant flow path 20 has a first flow path 22 through which the refrigerant flows via the automatic transmission 30, and a second flow path 23 through which the refrigerant flows without passing through the automatic transmission 30. The first flow path 22 and the second flow path 23 are switched by a valve 21. The first flow path 22 is a flow path through which the automatic transmission 30 can transfer heat to and from the refrigerant, and the second flow path 23 is a flow path through which the automatic transmission 30 cannot transfer heat to and from the refrigerant.

[0012] In this embodiment, the valve 21 is a thermo valve, and when the temperature of the refrigerant flowing through the refrigerant flow path 20 rises above a predetermined switching temperature, the refrigerant flow path 20 is switched from the second flow path 23 to the first flow path 22. The switching temperature range is, for example, 78 degrees Celsius or higher and 85 degrees Celsius or lower.

[0013] The automatic transmission 30 automatically changes the gear ratio. The power of the engine 10 is converted into a desired torque by the automatic transmission 30 and transmitted to the drive wheels of the vehicle 100.

[0014] The oil temperature sensor 31 measures the temperature of the hydraulic oil in the automatic transmission 30. The oil temperature sensor 31 is disposed in a flow path through which the hydraulic oil in the automatic transmission 30 flows.

[0015] 2 is a schematic diagram showing an overview of the fault detection device 40. The fault detection device 40 is made up of a computer configured with a central processing unit (CPU), RAM, ROM, etc., and the functions of an acquisition unit 43, a determination unit 44, and a judgment unit 45 are realized by the CPU 41 executing a program pre-installed in a storage unit 42 of the fault detection device 40. However, some or all of the functions of these units may be realized by hardware circuits.

[0016] The acquisition unit 43 acquires the oil temperature measured by the oil temperature sensor 31. In this embodiment, the acquisition unit 43 also acquires the cumulative time during which there is a difference between the rotation speed of the engine 10 and the input rotation speed of the automatic transmission 30. More specifically, the acquisition unit 43 acquires the cumulative time during which there is no lockup state. The lockup state is a state in which the difference between the rotation speed of the engine 10 and the input rotation speed of the automatic transmission 30 is zero. The acquisition unit 43 acquires a lockup signal that controls the engine 10 and the automatic transmission 30 to enter a lockup state, and measures the time during which the lockup state is not entered.

[0017] The determination unit 44 determines the preconditions and the failure conditions according to the flow path of the refrigerant flow path 20. More specifically, the determination unit 44 determines the preconditions and the failure conditions according to whether the flow path of the refrigerant flow path 20 is the first flow path 22 or the second flow path 23. The preconditions are conditions used by the acquisition unit 43 to determine whether or not to acquire the oil temperature from the oil temperature sensor 31. The failure conditions are conditions used to determine whether the oil temperature sensor 31 has failed.

[0018] In this embodiment, a prerequisite is that the cumulative time during which there is a difference between the rotation speed of the engine 10 and the input rotation speed of the automatic transmission 30 since the engine 10 started, i.e., the cumulative time during which the automatic transmission 30 is not in a lock-up state, is equal to or greater than a predetermined threshold time. When the automatic transmission 30 is not in a lock-up state, the amount of heat received by the automatic transmission 30 is greater than when the automatic transmission 30 is in a lock-up state. Therefore, when the cumulative time during which the automatic transmission 30 is not in a lock-up state is equal to or greater than the threshold, the oil temperature does not rise sufficiently, which can prevent erroneous determination.

[0019] Furthermore, the determination unit 44 determines the threshold time when the flow path of the refrigerant flow path 20 is the first flow path 22 to be shorter than the threshold time when the flow path of the refrigerant flow path 20 is the second flow path 23. When the oil temperature is high, a lock-up state is more likely to occur than when the oil temperature is low, so when the flow path of the refrigerant flow path 20 is the first flow path 22, the cumulative time during which the lock-up state is not reached is more likely to be shorter than when the flow path of the refrigerant flow path 20 is the second flow path 23. Therefore, by determining the threshold time to be shorter, it is possible to prevent the preconditions from being not met and to reduce the frequency of determining whether or not the failure conditions are met.

[0020] In this embodiment, the failure condition is that the oil temperature is lower than a predetermined threshold temperature. The determination unit 44 determines the threshold temperature when the refrigerant flow path 20 is the second flow path 23 to be lower than the threshold temperature when the refrigerant flow path 20 is the first flow path 22. When the refrigerant flow path 20 is the second flow path 23, the amount of heat exchanged by the automatic transmission 30 is smaller than when the refrigerant flow path 20 is the first flow path 22, and the likelihood of the oil temperature being lower is higher, so the threshold temperature is determined to be lower. This makes it possible to prevent erroneous determination of a failure.

[0021] When the preconditions are met, the determination unit 45 determines whether the failure conditions are met, and when the failure conditions are met, it determines that there is a failure in the oil temperature sensor 31. That is, in this embodiment, the determination unit 45 determines that there is a failure in the oil temperature sensor 31 when the accumulated time during which there is no lockup state is equal to or greater than the threshold time and the oil temperature is less than the threshold temperature.

[0022] 3 is a flowchart showing an example of the failure detection process. The failure detection process is a process for determining whether or not there is a failure in the oil temperature sensor 31. This process is executed, for example, immediately after the engine 10 is started.

[0023] In step S100, the fault detection device 40 determines whether the flow path of the refrigerant flow path 20 is the first flow path 22. More specifically, the fault detection device 40 determines whether the temperature of the refrigerant is equal to or higher than the switching temperature. If the temperature of the refrigerant is equal to or higher than the switching temperature, i.e., if the flow path of the refrigerant flow path 20 is the first flow path 22, the process proceeds to step S110. On the other hand, if the temperature of the refrigerant is lower than the switching temperature, i.e., if the flow path of the refrigerant flow path 20 is the second flow path 23, the process proceeds to step S115.

[0024] <When the flow path of the refrigerant flow path 20 is the first flow path 22> In step S110, the determination unit 44 determines a first precondition and a first fault condition. In this embodiment, the determination unit 44 determines the first precondition to be that the cumulative time during which the engine is not in a lockup state is equal to or greater than a first threshold time. The first threshold time is, for example, 100 seconds. The determination unit 44 also determines the first fault condition to be that the oil temperature is less than a first threshold temperature. The first threshold temperature is, for example, 60 degrees.

[0025] In step S120, the judgment unit 45 judges whether or not the first precondition determined by the decision unit 44 in step S110 is satisfied. More specifically, the judgment unit 45 judges whether or not the cumulative time during which the vehicle is not in a lockup state acquired by the acquisition unit 43 is equal to or greater than a first threshold time. If the cumulative time during which the vehicle is not in a lockup state is equal to or greater than the first threshold time, that is, if the first precondition is satisfied, the fault detection device 40 proceeds to the process of step S130. On the other hand, if the cumulative time during which the vehicle is not in a lockup state is less than the first threshold time, that is, if the first precondition is not satisfied, the fault detection device 40 returns to the process of step S100. That is, the fault detection device 40 repeats the processes of steps S100 to S120 until the first precondition is satisfied.

[0026] In step S130, the acquisition unit 43 acquires the oil temperature measured by the oil temperature sensor 31.

[0027] In step S140, the determination unit 45 determines whether the first fault condition determined by the determination unit 44 in step S110 is satisfied. More specifically, the determination unit 45 determines whether the oil temperature acquired in step S130 is lower than the first temperature. If the oil temperature is lower than the first threshold temperature, that is, if the first fault condition is satisfied, the fault detection device 40 proceeds to the process of step S150 and determines that the automatic transmission 30 has a fault. The process of step S150 is also referred to as the "determination process." On the other hand, if the oil temperature is equal to or higher than the first threshold temperature, that is, if the first fault condition is not satisfied, the fault detection device 40 returns to the process of step S100. That is, the fault detection device 40 repeats the processes of steps S100 to S140 until the first precondition is satisfied.

[0028] <When the flow path of the refrigerant flow path 20 is the second flow path 23> In step S115, the determination unit 44 determines a second precondition and a second fault condition. In this embodiment, the determination unit 44 determines the second precondition to be that the cumulative time during which the lockup state is not reached is equal to or greater than a second threshold time. The second threshold time is longer than the first threshold time, for example, 200 seconds. The determination unit 44 also determines the second fault condition to be that the oil temperature is lower than a second threshold temperature. The second threshold temperature is lower than the first threshold temperature. The second threshold temperature is, for example, 40 degrees. Steps S110 and S115 are collectively referred to as the "determination process."

[0029] In step S125, the judgment unit 45 judges whether or not the second precondition determined by the decision unit 44 in step S115 is satisfied. More specifically, the judgment unit 45 judges whether or not the cumulative time during which the vehicle is not in a lockup state acquired by the acquisition unit 43 is equal to or greater than a second threshold time. If the cumulative time during which the vehicle is not in a lockup state is equal to or greater than the second threshold time, that is, if the second precondition is satisfied, the fault detection device 40 proceeds to the process of step S135. On the other hand, if the cumulative time during which the vehicle is not in a lockup state is less than the second threshold time, that is, if the second precondition is not satisfied, the fault detection device 40 returns to the process of step S100. That is, the fault detection device 40 repeats the processes of steps S100 to S125 until the second precondition is satisfied.

[0030] In step S135, the acquisition unit 43 acquires the oil temperature measured by the oil temperature sensor 31. Note that steps S130 and S135 are collectively referred to as an "acquisition step."

[0031] In step S145, the determination unit 45 determines whether the second fault condition determined by the determination unit 44 in step S115 is satisfied. More specifically, the determination unit 45 determines whether the oil temperature acquired in step S135 is lower than the second temperature. If the oil temperature is lower than the second threshold temperature, that is, if the second fault condition is satisfied, the fault detection device 40 proceeds to the process of step S150 and determines that the automatic transmission 30 has a fault. On the other hand, if the oil temperature is equal to or higher than the second threshold temperature, that is, if the second fault condition is not satisfied, the fault detection device 40 returns to the process of step S100. That is, the fault detection device 40 repeats the processes of steps S100 to S145 until the second precondition is satisfied.

[0032] According to the fault detection device 40 of the present embodiment described above, the prerequisites and fault conditions are determined according to the flow path of the refrigerant flow path 20. The determination unit 44 determines the second threshold temperature for the fault condition when the flow path of the refrigerant flow path 20 is the second flow path 23 to be a temperature lower than the first threshold temperature for the fault condition when the flow path of the refrigerant flow path 20 is the first flow path 22. Therefore, even when the flow path of the refrigerant flow path 20 is the second flow path 23, the amount of heat received by the automatic transmission 30 is small, and the oil temperature does not rise sufficiently, it is possible to prevent an erroneous determination that the oil temperature sensor 31 is faulty.

[0033] Furthermore, because the precondition includes the cumulative time during which a difference exists between the engine 10 rotation speed and the automatic transmission 30 input rotation speed, the determination unit 45 can determine whether a failure condition exists after the oil temperature has risen sufficiently, thereby reducing erroneous determinations. Furthermore, a lockup state is more likely to occur when the oil temperature is high than when the oil temperature is low. That is, when the refrigerant flow path 20 is the first flow path 22, the cumulative time during which the lockup state is not existed is more likely to be shorter than when the refrigerant flow path 20 is the valve 21. The determination unit 44 determines the first threshold time of the precondition when the refrigerant flow path 20 is the first flow path 22 to be shorter than the second threshold time of the precondition when the refrigerant flow path 20 is the second flow path 23. This prevents the precondition from being met and reduces the frequency of determining whether a failure condition exists.

[0034] B. Second embodiment: In the second embodiment, the first precondition in the fault detection process when the flow path of the refrigerant flow path 20 is the first flow path 22 is different from the first precondition in the first embodiment, but the other steps are the same as those in the first embodiment. Furthermore, since the configuration of the fault detection device 40 in the second embodiment is the same as the configuration of the fault detection device 40 in the first embodiment, a description of the configuration of the fault detection device 40 will be omitted.

[0035] In step S110, the decision unit 44 determines that the cumulative time during which there is a difference between the rotation speed of the engine 10 and the input rotation speed of the automatic transmission 30 is equal to or greater than the first threshold time, and sets the first threshold time to 0 seconds. That is, in the first precondition in the second embodiment, the cumulative time during which the lockup state is not in effect is not taken into account.

[0036] The determination unit 44 determines the first precondition to be that the elapsed time since the engine 10 was started is equal to or greater than a third threshold time, the cumulative time that the accelerator pedal of the vehicle 100 has been on since the engine 10 was started is equal to or greater than a fourth threshold time, and the cumulative mileage of the vehicle 100 since the engine 10 was started is equal to or greater than a first threshold distance. The third threshold time is longer than the first threshold time and the fourth threshold time, for example, 1100 seconds. The fourth threshold time is longer than the first threshold time, for example, 250 seconds. The first threshold distance is, for example, 7 km. In the second embodiment, the acquisition unit 43 acquires the elapsed time since the engine 10 was started, the cumulative time that the accelerator pedal has been on, and the cumulative mileage of the vehicle 100. The determination unit 45 determines whether the first precondition is satisfied using the information acquired by the acquisition unit 43.

[0037] According to the fault detection device 40 of the second embodiment described above, the determiner 44 determines the first precondition to be that the cumulative time during which the vehicle is not in a lockup state is 0 seconds or more, the elapsed time since the engine 10 was started is equal to or greater than the third threshold time, the cumulative time during which the vehicle 100 has had the accelerator pedal on since the engine 10 was started is equal to or greater than the fourth threshold time, and the cumulative mileage of the vehicle 100 since the engine 10 was started is equal to or greater than the first threshold distance. Therefore, even if the vehicle 100 is driven in a normal temperature range that is not low temperature and the vehicle 100 enters a lockup state, it is possible to suppress a decrease in the frequency of determining whether or not the fault condition is met due to the precondition not being met, and to suppress an erroneous determination that the oil temperature sensor 31 is faulty.

[0038] C. Other Embodiments: (C1) In the above-described embodiment, the fault detection device 40 determines whether the flow path of the refrigerant flow path 20 is the first flow path 22 based on whether the temperature of the refrigerant is equal to or higher than the switching temperature of the valve 21. However, the present invention is not limited to this. The fault detection device 40 may determine whether the flow path of the refrigerant flow path 20 is the first flow path 22 based on, for example, the detection result of a sensor that can detect whether the refrigerant is flowing through the first flow path 22 or the presence or absence of a signal accompanying the switching of the valve 21.

[0039] (C2) In the above-described embodiment, the determination unit 44 may determine each threshold value for the prerequisite condition and the fault condition using a function or table that defines a relationship between each threshold value for the prerequisite condition and the fault condition and the refrigerant temperature in advance. Each threshold value for a high refrigerant temperature is determined to be equal to or less than the corresponding threshold value for a low refrigerant temperature.

[0040] (C3) In the above-described embodiment, the precondition includes that the cumulative time during which a difference occurs between the rotation speed of the engine 10 and the input rotation speed of the automatic transmission 30 after the engine 10 is started is equal to or greater than a predetermined threshold time. However, the precondition does not necessarily include that the cumulative time during which a difference occurs between the rotation speed of the engine 10 and the input rotation speed of the automatic transmission 30 after the engine 10 is started is equal to or greater than a threshold time.

[0041] (C4) In the second embodiment described above, the first precondition includes that the time elapsed since the start of the engine 10 is equal to or greater than the third threshold time, that the cumulative time that the accelerator of the vehicle 100 has been on since the start of the engine 10 is equal to or greater than the fourth threshold time, and that the cumulative mileage of the vehicle 100 since the start of the engine 10 is equal to or greater than the first threshold distance. However, the first precondition may include any one or more of that the time elapsed since the start of the engine 10 is equal to or greater than the third threshold time, that the cumulative time that the accelerator of the vehicle 100 has been on since the start of the engine 10 is equal to or greater than the fourth threshold time, and that the cumulative mileage of the vehicle 100 since the start of the engine 10 is equal to or greater than the first threshold distance.

[0042] (C5) In the second embodiment described above, the second precondition may include any one or more of: the elapsed time since engine 10 was started is equal to or greater than a fifth threshold time; the cumulative time that vehicle 100 has been in an accelerator-on state since engine 10 was started is equal to or greater than a sixth threshold time; and the cumulative mileage of vehicle 100 since engine 10 was started is equal to or greater than a second threshold distance. In this case, it is preferable that determination unit 45 determines the fifth threshold time to be equal to or greater than the third threshold time, the sixth threshold time to be equal to or greater than a fourth threshold time, and the second threshold distance to be equal to or greater than the first threshold distance. That is, determination unit 45 determines the thresholds in the second precondition when the refrigerant flow path 20 is the second flow path 23 to be equal to or greater than the thresholds in the first precondition when the refrigerant flow path 20 is the first flow path 22.

[0043] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0044] 10...engine, 20...refrigerant flow path, 21...valve, 22...first flow path, 23...second flow path, 30...automatic transmission, 31...oil temperature sensor, 40...fault detection device, 41...CPU, 42...storage unit, 43...acquisition unit, 44...decision unit, 45...determination unit, 100...vehicle, 110...engine system

Claims

1. A fault detection device for an oil temperature sensor that measures the temperature of hydraulic oil in an automatic transmission mounted on a vehicle, an acquisition unit that acquires the oil temperature measured by the oil temperature sensor; a determination unit that determines a precondition for determining whether or not to acquire the oil temperature and a failure condition for determining a failure of the oil temperature sensor, according to a refrigerant flow path that switches between a first flow path through which a refrigerant that cools the engine of the vehicle flows via the automatic transmission and a second flow path through which the refrigerant flows without passing through the automatic transmission; a determination unit that determines whether the failure condition is satisfied when the prerequisite is satisfied, and determines that the oil temperature sensor has a failure when the failure condition is satisfied; the fault condition includes the oil temperature being below a threshold temperature; The determination unit determining the precondition when the flow path is the first flow path and the precondition when the flow path is the second flow path to be different conditions; A fault detection device that determines the threshold temperature when the flow path is the second flow path to be lower than the threshold temperature when the flow path is the first flow path.

2. 2. The fault detection device according to claim 1, the precondition includes that a cumulative time during which a difference occurs between the engine rotation speed and the input rotation speed of the automatic transmission after the engine is started is equal to or longer than a predetermined first threshold time, The failure detection device, wherein the determination unit determines the first threshold time when the flow path is the first flow path to be shorter than the first threshold time when the flow path is the second flow path.

3. 3. The fault detection device according to claim 2, When the flow path is the first flow path, the determination unit determining the first threshold time to be 0; A fault detection device that determines the preconditions to include one or more of the following: the elapsed time since the engine was started is equal to or greater than a predetermined second threshold time; the cumulative time that the vehicle has been in an accelerator-on state since the engine was started is equal to or greater than a predetermined third threshold time; and the cumulative mileage of the vehicle since the engine was started is equal to or greater than a predetermined threshold distance.

4. A method for detecting a fault in an oil temperature sensor that measures the temperature of hydraulic oil in an automatic transmission mounted on a vehicle, comprising: an acquisition step of acquiring an oil temperature measured by the oil temperature sensor; a determining step of determining a precondition for determining whether or not to acquire the oil temperature and a failure condition for determining a failure of the oil temperature sensor, according to a refrigerant flow path that switches between a first flow path through which a refrigerant for cooling an engine of the vehicle flows via the automatic transmission and a second flow path through which the refrigerant flows without passing through the automatic transmission; a determination step of determining whether the failure condition is satisfied when the prerequisite condition is satisfied, and determining that the oil temperature sensor has a failure when the failure condition is satisfied, the fault condition includes the oil temperature being below a threshold temperature; In the determining step, determining the precondition when the flow path is the first flow path and the precondition when the flow path is the second flow path to be different conditions; A fault detection method, wherein the threshold temperature when the flow path is the second flow path is determined to be lower than the threshold temperature when the flow path is the first flow path.

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