Powertrain Management (CNG)

DTC P0125

Circuit Description

The engine coolant temperature (ECT) sensor is a thermistor. The ECT thermistor has high resistance when cold and low resistance when hot. The powertrain control module (PCM) supplies about 5 volts on the ECT signal circuit. The PCM also supplies a ground circuit to the ECT sensor. When the ECT resistance is high the ECT signal voltage remains near the supplied 5 volts. As the ECT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The PCM and the Alternative Fuels ECM monitor the ECT signal circuit voltage in order to determine the temperature of the engine.


This DTC sets when the engine coolant temperature is less than the temperature necessary to enable AF ECM Closed Loop fuel control.


Conditions for Running the DTC

* The engine is operating on alternative fuel.
* DTCs P0112, P0113, P0117, or P0118 are not set.
* This DTC has not tested this ignition cycle.
* The engine has been operating for greater than 8.5 minutes.
* The vehicle speed is greater than 8 km/h (5 mph).
* The desired gas flow is greater than 0.5 grams per second.
* The intake air temperature (IAT) is greater than -15°C (5°F).
* The ECT at engine start-up is between -17 to +33°C (1-91°F).



Conditions for Setting the DTC

The Alternative Fuels ECM monitors an engine coolant temperature that has NOT increased above the start-up temperature by 25°C (77°F).


Action Taken When the DTC Sets




* The Alternative Fuels ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
* The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.



Conditions for Clearing the MIL/DTC




* The Last Test Failed status will clear when the DTC runs and passes.
* The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
* The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
* All DTC Information can be cleared with a scan tool.



Diagnostic Aids

The most likely cause of this DTC is an improperly operating engine coolant system and thermostat. Refer toEngine Fails To Reach Normal Operating Temperature in Engine Cooling in the C/K Pickup Service Manual in order to verify proper cooling system and thermostat operation.


Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. Ensure that the engine can reach proper operating temperature in a timely manner before continuing with ECT sensor diagnosis.
3. This step determines if the Alternative Fuels ECM can monitor an open ECT signal circuit.
4. This step determines if the Alternative Fuels ECM can monitor a shorted ECT signal circuit.
5. This step determines if the ECT sensor is accurately monitoring engine coolant temperature.