Thermostat malfunction detection device

A thermostat malfunction detection device that detects a stuck open state of a thermostat that opens and closes a cooling water flow channel between a radiator including a passing air amount changing unit that increases and decreases a passing air amount and an engine, according to a temperature of a cooling water, comprising:

• an estimated water temperature calculation module that is configured to calculate an estimated water temperature of the cooling water based on an operation state of the engine;
• an actual water temperature detection unit that is configured to detect an actual water temperature of the cooling water;
• a first determination module that is configured to establish preliminary determination based on an amount of change of a difference between the estimated water temperature and the actual water temperature within a predetermined period during cooling water temperature rising is equal to or larger than a first threshold value;
• a passing air amount control module that is configured to control the passing air amount changing unit to increase a passing air amount, based on establishment of the preliminary determination by the first determination module;
• and
• a second determination module that is configured to establish malfunction determination, based on the difference between the estimated water temperature and the actual water temperature being equal to or larger than a second threshold value, after the passing air amount control module has increased the passing air amount.