Thermostat Monitor
The thermostat monitor is designed to verify correct thermostat operation. This monitor is executed once per drive cycle and has a monitor run duration of 300-800 seconds. If a malfunction occurs, a DTC P0125 or P0128 is set and the MIL is illuminated.
The monitor checks the engine coolant temperature (ECT) or cylinder head temperature (CHT) sensor to warm up in a predictable manner when the engine is generating sufficient heat. A timer is initialized while the engine is at moderate load and the vehicle speed is above a calibrated limit. The target timer value is based on ambient air temperature at start-up. If the timer exceeds the target time and ECT or CHT has not warmed up to the target temperature, a malfunction is indicated. The test runs if the start-up intake air temperature from the IAT sensor is at, or below the target temperature. A 2-hour engine off soak time is also required to enable the monitor and to prevent erasing of any pending DTC during a hot soak. This soak time feature also prevents false-passes of the monitor when the engine coolant temperature rises after the engine is turned off during a short engine off soak period.
The target temperature is calibrated to the thermostat regulating temperature minus 11°C (20°F). For a typical 90°C (195°F) thermostat, the warm-up temperature would be calibrated to 79°C (175°F). Some vehicle calibrations may lower the target temperature below 27°C (50°F) for vehicles that do not warm-up to thermostat regulating temperatures in the 11°C (20°F) to 27°C (50°F) ambient temperature range.
- Inputs: ECT or CHT, IAT, engine LOAD (from MAF sensor) and vehicle speed input.
Typical monitor entry conditions:
- vehicle speed greater than 24 km/h (15 mph)
- intake air temperature at start-up is between -7°C (20°F) and target thermostat temperature
- engine load greater than 30%
- engine off (soak) time greater than 2 hours
- Output: MIL.