Patent ID: 8770834

Claim:
A thermostat diagnostic apparatus comprising: a cooling medium temperature sensor; an engine operating condition sensor; and a malfunction diagnosing device configured to conduct a thermostat stuck-open malfunction diagnosis of a thermostat provided in a coolant flow passage of an engine installed in a mobile body based on a comparison of a real cooling medium temperature detected by the cooling medium temperature sensor and an estimated cooling medium temperature estimated based on an engine operating condition of the engine detected by the engine operating condition sensor, the malfunction diagnosing device determining that the thermostat is stuck in an open state upon determining that either the estimated cooling medium temperature or the real cooling medium temperature exceeds a prescribed reference value during a period in which an increased heat exchange rate condition of a radiator is satisfied continuously; the malfunction diagnosing device being configured to determine that the increased heat exchange rate condition of the radiator is satisfied based on a prescribed speed of the mobile body such that the increased heat exchange rate condition of the radiator is determined to be satisfied when a speed of the mobile body is higher than the prescribed speed, and the malfunction diagnosing device being further configured to variably set the prescribed speed based on an engine rotational speed of the engine such that as the engine rotational speed becomes higher, the prescribed speed is set to a smaller value.