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

Engine Coolant Temperature (ECT) Sensor:

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the left cylinder head. Low coolant temperature produces a high resistance (100,000 ohms at -38°C/-39°F) while high temperature causes low resistance (70 ohms at 130°C/266°F).

The PCM supplies a 5.0 volt signal to the engine coolant temperature sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the engine is cold. The voltage will be low when the engine is hot. By measuring the voltage, the PCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.

The scan tool displays engine coolant temperature in degrees. After engine start up, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. When the PCM detects a malfunction in the ECT sensor circuit, the following DTCs will set:
^ DTC P0117 ECT Sensor Circuit Low Voltage.
^ DTC P0118 ECT Sensor Circuit High Voltage.
^ DTC P0125 ECT Excessive Time To Closed Loop.
^ DTC P1114 ECT Sensor Circuit Intermittent Low Voltage.
^ DTC P1115 ECT Sensor Circuit Intermittent High Voltage.

Service Category Specifications contains a table to check for sensor resistance values relative to temperature.