Thermostat Monitor

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.

Thermostat Monitor is designed to verify proper thermostat operation. This monitor only applies to certain applications beginning with the 2000 model year and replaces "Insufficient temperature for closed-loop test" (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 thermostat monitor a DTC P0125 will be set and MIL will be illuminated. The monitor checks Engine Coolant Temperature (ECT) or Cylinder Head Temperature (CHT) sensor to warm up in a predictable manner when engine is generating sufficient heat. A timer is started while engine is at moderate load and vehicle speed is above a calibrated limit. Target timer value is based on ambient air temperature at start-up. If timer exceeds the target time and ECT/CHT has not warmed up to target temperature, a malfunction is indicated. The test runs if start-up intake air temperature from IAT sensor is at, or below target temperature. A 2-hour engine off soak time is required to erase a pending or confirmed DTC. This soak time feature prevents false-passes of monitor when the engine coolant temperature rises after engine is turned off during a short engine off soak period. The target temperature will be calibrated to thermostat regulating temperature minus 20°F (11°C). For a typical 195°F (90°C) thermostat, warm-up temperature would be calibrated to 175°F (79°C). For the 2003 model year, some vehicle calibrations may lower the target temperature when ambient temperature is below 50°F (10°C). This lower target temperature is for vehicles that do not warm-up to the thermostat regulating temperature when ambient temperature is 20-50°F (-7-10°C).

Inputs

ECT or CHT, IAT, engine LOAD (from MAF sensor) and vehicle speed input. Typical monitor entry conditions are as follows:
- Vehicle speed more than 15 MPH.
- Intake air temperature at start-up is between 20°F (-7°C) and target thermostat temperature.
- Engine load more than 30 percent.
- Engine off (soak) time more than 2 hours.

Output

Malfunction Indicator Lamp (MIL)