Engine Temperature Sensor: Description and Operation

Engine Coolant Temperature Sensor (ECT Sensor)
Incorporated with coolant temp. gauge and installed to thermostat case, this sensor measures the temperature of the engine coolant and converts its change into that in resistance through the thermistor like the air temperature sensor.

That is, as coolant temperature lowers, resistance increases and as it rises, resistance decreases.

By monitoring the resistance of the coolant temperature sensor, ECM (PCM) detects the engine coolant temperature and that affects most systems under the control of ECM (PCM).