The Thermostat Monitor is designed to verify proper thermostat operation. This monitor will be phased in on certain applications beginning with the 2000 model year and replaces the original "Insufficient temperature for closed-loop test" (Diagnostic Trouble Code (DTC) P0125). This monitor will be executed once per drive cycle, after a two hour, engine-off soak period. If a malfunction is indicated by the thermostat monitor a diagnostic trouble code P0125 will be set and the malfunction indicator lamp will be illuminated.
The monitor checks to see if the engine is being operated in a manner that is generating sufficient heat. While the engine is at moderate load (greater than 30%) and the vehicle is moving (greater than 15 mph/24 km ), the Engine Coolant Temperature (ECT) or Cylinder Head Temperature (CHT) should warm up in a predictable manner, therefore, a timer is incremented. The target timer value is based on ambient air temperature at start-up. If the timer exceeds the target time and the ECT or CHT has not warmed up to the target temperature, a malfunction is indicated.
The target temperature will be calibrated to the thermostat regulating temperature minus 20°F (11°C) . For a typical 195°F (90°C) thermostat, the warm-up temperature would be calibrated to 175°F (79°C) .
Inputs: Engine Coolant Temperature (ECT) or Cylinder Head Temperature (CHT) , Intake Air Temperature (IAT) , engine LOAD (from Mass Air Flow (MAF) sensor) and vehicle speed input.