Thermostat Monitor

THERMOSTAT MONITOR

Thermostat Monitor:

The thermostat monitor is designed to verify proper thermostat operation. This monitor is executed once per drive cycle and has a monitor run duration of 300 - 800 seconds. If a problem occurs, diagnostic trouble code (DTC) P0128 is set and the malfunction indicator lamp (MIL) will be illuminated.

The monitor checks the engine coolant temperature by monitoring the cylinder head temperature. (CHT) sensor to warm up in a predictable manner when the engine is generating sufficient heat. A timer is incremented while the engine is at moderate load and the vehicle speed is above a calibrated limit. The target timer value is based on the ambient air temperature at start-up. If the timer exceeds the target time and the CHT has not warmed up to the target temperature, a problem is indicated. The test runs if the start-up intake air temperature from the intake air temperature (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).

1. Inputs: CHT, IAT, engine load from mass air flow (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

2. Output: MIL