Theory Of Operation

The Thermostat is considered malfunctioning if the coolant temperature does not reach the temperature required to enable other diagnostics, or the coolant temperature does not reach a warmed up temperature within 11.0°C (20°F) of the Thermostat regulating temperature. The Powertrain Control Module (PCM) creates a model of the engine coolant temperature warm up cycle, based on the engine coolant temperature at start up, ambient temperature, vehicle speed and engine speed. The modeled engine coolant temperature is compared to the Engine Coolant Temperature (ECT) Sensor reading. If the engine actual temperature reaches the calibrated temperature threshold before the model, the diagnostic passes and completes.

If the model reaches the calibrated temperature threshold before the engine actual temperature, a pass/fail determination is made based on the difference between the model and actual values. The diagnostic is considered passing if the difference between the engine actual temperature and model is acceptable when the model is greater than the engine but represents the slowest possible warm-up for a "good" thermostat.