Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor, located in the intake manifold below the thermostat housing, is a variable resistor that measures the temperature of the engine coolant. See Fig 1. The Powertrain Control Module (PCM) supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When the engine coolant temperature is low, the sensor resistance is high. When the engine coolant temperature is high, the sensor resistance is low. The PCM uses this High Side Coolant Rationality test to determine if the ECT input is skewed high. The internal clock of the PCM will record the amount of time the ignition is OFF. At restart the PCM will compare the temperature difference between the ECT and the Intake Air Temperature (IAT). Before failing this test, the PCM will perform a calculation to determine the presence of an engine block heater. If the PCM detects that the temperature difference between the ECT and the IAT is not within the calibrated range after the ignition OFF time, DTC P0116 sets.

When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance decreases. With low/high sensor resistance, the PCM detects a low/high voltage on the ECT signal circuit. With higher/lower sensor resistance, the PCM detects a higher/lower voltage on the ECT signal circuit. If the PCM detects an excessively high/low ECT signal voltage, which is a low/high temperature indication, a DTC sets.

Fig 1: Identifying Engine Coolant Temperature Sensor

Courtesy of GENERAL MOTORS CORP.