OBD Operation Summary for Fiesta - MotorCraftService.com

Thermostat MonitorThe coolant thermostat monitor is designed to detect a thermostat that is slow to open or is stuck open. It is based on thecomparison of the measured Engine Coolant Temperature (ECT) sensor signal and the calculated ECT model. The ECTmodel calculation is depending on engine load/speed and the intake air temperature.A malfunctioning coolant thermostat is detected if the calculated ECT model has exceeded the thermostat openingtemperature and the measured ECT sensor signal remains below a threshold (the highest diagnostic enabletemperature).To prevent false DTCs, conditions for low load, long deceleration duration and Intake Air Temperature (IAT) during themonitoring period are checked. If the monitoring conditions are met, the thermostat DTC is set; otherwise the thermostatmonitor is suspended for the current driving cycle.Thermostat Monitor Method:A comparison between the measured coolant temperature and the “warmed-up temperature” is done after a specifictime interval. The interval itself is based on the coolant temperature model.As soon as the model temperature exceeds the thermostat opening temperature and all other monitoring conditions arefulfilled at the same time, a valid diagnosis occurs.At that time, if the measured coolant temperature is higher than warmed-up temperature, the thermostat is considered tobe a normal thermostat. If the measured coolant temperature is lower than warmed-up temperature, the thermostat isconsidered to be stuck open.CONTINENTAL REVISED: SEPTEMBER 17, 2010 PAGE 59 OF 111