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°F.) while high temperature causes low resistance (70 ohms at 266°F.). The PCM supplies a voltage (about 5.0V) through a pull up resistor to the PCM's ECT signal circuit and measures the voltage. When the engine coolant is cold, the sensor (thermistor) resistance is high, therefore the PCM will measure a high signal voltage. As the engine coolant warms, the sensor resistance becomes less, and the ECT signal voltage measured at the PCM drops. With a fully warmed up engine, the ECT signal voltage should measure about 1.5-2.0 volts. By measuring the voltage, the PCM calculates the engine coolant temperature and engine coolant temperature affects most systems the PCM controls.
A technician's scan tool displays engine coolant temperature in degrees. After engine start up, the temperature should rise steadily to about 195°F (90°C), then stabilize and the 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 hard or even an intermittent fault in the engine coolant temperature sensor circuit will set a hard Diagnostic Trouble Code (DTC).
Most ECT sensor also contain another circuit which is used to operate the engine coolant temperature gauge (if equipped) located in the instrument panel.