Circuit/System Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the signal circuit and a ground for the low reference circuit. When the engine coolant temperatures are low, the resistance is high. When the engine coolant temperatures are high, the resistance is low. The ECM uses this high side coolant rationality test to determine if the ECT input is skewed high. The ECM will record the amount of time the engine is OFF. At restart, the ECM will compare the temperature difference between the ECT and the intake air temperature (IAT). Before failing this diagnostic, the ECM will perform the calculation to determine the presence of a block heater.