P0125

Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. When the vehicle is first started, the PCM controls fuel delivery in open loop, ignoring the heated oxygen sensor (HO2S) 1 signal and calculating air/fuel ratio based on inputs from the engine coolant temperature, Throttle position, and mass air flow sensors. The PCM will begin using the HO2S 1 signal for controlling fuel delivery (Closed Loop) when the following conditions have been met:
^ The HO2S 1 signal is varying, indicating that the sensor is hot enough to operate properly.
^ The engine coolant temperature sensor indicates coolant temperature above 10°C (50°F).
^ Time since startup is at least 10 seconds for a warm engine or 50 seconds for a cold engine.

Conditions for Running the DTC
^ No IAT or ECT sensor DTCs present
^ Intake air temperature is greater than -7°C (19°F).
^ Engine coolant temperature is greater than -40°C (-40°F).
^ Start up coolant temperature less than 50°C (122°F).

Conditions for Setting the DTC
An excessive amount of time passes before engine coolant temperature reaches the Closed Loop operation (10°C/50°F) threshold. The amount of time ranges between 2 and 13 minutes depending on engine coolant temperature at startup and the amount of air cycled through the engine since startup.

Action Taken When the DTC Sets
^ The PCM illuminates the Malfunction Indicator Lamp on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure records.

Conditions for Clearing the MIL/DTC
^ The PCM turns the MIL Oft after three consecutive drive trips that the diagnostic runs and does not fail.
^ A last test failed (current DTC) clears when the diagnostic runs and does not fail.
^ A History DTC clears after forty consecutive warm-up cycles, if this or any Other emission related diagnostic does not report any failures.
^ Use a scan tool in order to clear the MIL/DTC.
^ Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Diagnostic Aids

Notice: Use the Connector Test Adapter Kit J 35616-A for any test that requires probing the following items:
^ The PCM harness connectors
^ The electrical center fuse/relay cavities
^ The component terminals
^ The component harness connector

Using this kit will prevent damage caused by the improper probing of connector terminals.

The DTC P0125 diagnostic test will not run if the engine is started while the coolant temperature is less than 0°C (32°F) or greater than 21°C (70°F). If it is not possible to operate the vehicle under the appropriate conditions, check the following items:
^ Cooling system coolant level.
^ Thermostat operation.
^ Cooling fans operation.
Refer to Electric Cooling Fan Diagnosis. Component Tests and General Diagnostics
^ Check for high resistance in wiring related to the ECT sensor. Also, check for poor connections at ECT sensor and PCM.
^ DTC P0125 set may indicate a skewed ECT sensor. Comparing the engine coolant temperature displayed on a scan tool with actual coolant temperature measured with a thermometer may isolate this condition. If the displayed engine coolant temperature is not close to the actual coolant temperature, and the ECT circuit/connections check out OK, replace the ECT sensor.

If DTC P0125 cannot be duplicated, the information included in the Fail Records data can be useful in determining vehicle mileage since the DTC was last set.


Test Description
An example test description follows:
7. Comparing the engine coolant temperature displayed on a scan tool with actual coolant temperature measured with a thermometer may isolate this condition
If the displayed engine coolant temperature is not close to the actual coolant temperature, replace the ECT sensor.
If the temperatures are close, the malfunction is intermittent; refer to Diagnostic Aids.