Coolant Temperature Sensor/Switch (For Computer): Description and Operation

Engine Coolant Temperature Sensor:

The Engine Coolant Temperature (ECT) sensor is a thermistor (a resistor which changes value based on temperature) immersed in the engine coolant stream. Low coolant temperature produces a high resistance while high temperature causes low resistance.

The Powertrain Control Module (PCM) supplies a 5 volt signal to the ECT through a resistor in the PCM and measures the voltage. The voltage will be high when the engine is cold and low when the engine is hot. By measuring the voltage. the PCM knows the engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.
A failure in the ECT circuit should set either a Diagnostic Trouble Code (DTC) 14 or 15. Remember these DTCs indicate a failure in the engine coolant temperature sensor circuit, so proper use of the chart will lead to either repairing a wiring problem or replacing the sensor to properly repair a problem.