Engine Coolant Temperature

The Engine Coolant Temperature (ECT) sensor sends engine temperature information to ECM. The ECM supplies 5 volts to engine coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When sensor is cold (internal resistance high), ECM monitors a high voltage which it interprets as a cold engine. As sensor warms (internal resistance decreases), voltage signal will decrease and ECM will interpret lower voltage as a warm engine.