DTC P0125: COOLANT TEMP RATIONALITY

Monitor & Set Conditions

Engine Coolant Temperature (ECT) sensor voltage is monitored when the engine is running, engine coolant temperature is -40 to 150°F (-40 to 65°C) and no Intake Air Temperature (IAT), ECT or battery temperature DTCs are set. ECT sensor voltage is also monitored when the engine is off with ignition on. Intake Air Temperature (IAT) must be less than 80°F (26°C), battery voltage must be 8-16 volts and no IAT, ECT or battery temperature DTCs are set. When the engine is running, DTC will set if coolant temperature reading does not change greater than 6°F within 10 minutes of starting the engine. With the engine off and ignition on, DTC will set if coolant temperature reading is greater than 140°F (60°C).

Possible Causes

• Wiring Harness
• ECT Sensor
• ECT Sensor - Engine Hot
• ECT

Testing

1. Turn ignition off. Disconnect ECT sensor harness connector. The ECT sensor is located at the front of the cylinder head, near the thermostat housing. Inspect ECT sensor connector for miswired, corroded, pushed out or damaged terminals. If terminals are not miswired, corroded, pushed out or damaged, go to next step. If terminals are miswired, corroded, pushed out or damaged, repair as necessary.

2. This test procedure will be invalid if the thermostat is stuck open. Turn ignition on. Measure engine temperature near ECT sensor using an external temperature measuring device. Ensure engine temperature is less than 124°F (50°C). Using DRBIII(R) scan tool, monitor ECT sensor reading. If both temperature readings are within 10 percent of each other, go to next step. If both temperature readings are not within 10 percent of each other, replace the ECT sensor. See appropriate REMOVAL, OVERHAUL & INSTALLATION article.

3. Continue monitoring engine temperature near ECT sensor using an external temperature measuring device and using a scan tool, continue monitoring ECT sensor reading. Start engine and allow it to reach a temperature greater than 180°F (82°C). If both temperature readings are within 10 percent of each other, replace and program the ECM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. If both temperature readings are not within 10 percent of each other, replace the ECT sensor. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. If engine temperature does not reach 180°F (82° C), diagnose cooling system malfunction.