Circuit/System Description

The engine coolant thermostat diagnosis is responsible to detect a delay in warming up of the engine coolant temperature. A stuck open thermostat is detected by monitoring the coolant temperature for a decrease below the diagnostic monitoring threshold during driving after the temperature once increases above the same threshold.

The purpose of this rationality diagnostics is to use the ECT sensor to determine if the engine coolant will heat up at the correct rate, and also meet the calibrated target temperatures under various operating conditions.