Thermostat Monitor

Thermostat Monitor:

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 and has a monitor run duration of 300-800 seconds. 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 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 incremented 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/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 two-hour engine off soak time is required to erase a pending or confirmed DTC. This soak time feature 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 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).

NOTE: Vehicles not part of the thermostat monitor phase-in will utilizes a 140°F (60°C) warm-up temperature.

1. Inputs: ECT or CHT, Intake Air Temperature (IAT), engine LOAD (from MAF sensor) and vehicle speed input.

Typical Monitor entry conditions:
^ Vehicle speed greater than 15 mph (24 km/h)
^ Intake Air temperature at start-up is between 20°F (-7°C) and target thermostat temperature
^ Engine load greater than 30%
^ Engine off (soak) time to clear pending or confirmed DTC is greater than a 2 hours

2. Output: Malfunction Indicator Lamp (MIL).