Circuit Description

The Engine Coolant Temperature (ECT) Sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM supplies a voltage on the signal circuit to the sensor. The output varies as the ECT increases. At normal engine operating temperature, the voltage will be between 1.5 and 5 volts at the ECT signal terminal.

When the vehicle is first started, the ECM monitors the ECT and determines if it reaches a specified temperature level within a predetermined time. The time is based on Intake Air Temperature (IAT) and the start-up ECT. The Malfunction Indicator Lamp (MIL) is illuminated and DTC P0128 is set when a stuck open thermostat prevents the engine coolant from reaching the predetermined operating conditions.