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

Engine Coolant Temperature (ECT) Sensor:

PURPOSE
The Engine Coolant Temperature (ECT) sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms at -40° C [-40° F]) while high temperature causes low resistance (70 ohms at 130° C [266° F]).

OPERATION
The Powertrain Control Module (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 engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.

The Tech 1 scan tool displays engine coolant temperature in degrees Celsius and Fahrenheit. 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. A fault in the engine coolant sensor circuit should set Diagnostic Trouble Code (DTC) P0117 or DTC P0118. The DTC charts also contain a table to check for sensor resistance values relative to temperature.