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

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

The PCM supplies a 5 volt signal to the ECT sensor 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 calculates the ECT. The scan tool displays the ECT in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F) and then stabilize when the thermostat opens. If the engine has not been run for several hours or overnight, the ECT and the Intake Air Temperature (IAT) displays should be close to each other.

The ECT affects most systems the PCM controls. A hard fault in the ECT sensor circuit should set a DTC P0117 or a DTC P0118. An intermittent fault should set a DTC P1114 or P1115. This section also contains a specification table to check the sensor resistance values relative to the temperature.