Engine coolant temperature sensor is located in the engine coolant passage of the cylinder head for detecting the engine coolant temperature.. It contains negative type themistor whoses resistance value is in inverse proportion to temperature. Resistance of watersensor that varies with temperature makes signal voltage of A/C Control Module change. In this way A/C Control Module detects temperature. Signal of water sensor is used for cold engine lockout control - When the driver operates the heater before the engine is warmed up enough, to prevent driver from unpleasantness of cold air. the heater control unit reduces blower motor speed until coolant temperature reaches the threshold value.