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 20°C (68°F).
- Time since start-up is at least 16 seconds for a warm engine or 50 seconds for a cold engine.

Conditions for Setting the DTC

- No IAT or ECT sensor DTCs present.
- Intake air temperature is greater than 10°C (50°F).
- Start-up engine coolant temperature is less than 10°C (50°F) and greater than -40°C (-40°F).
- Engine coolant temperature sufficient to allow Closed Loop operation (22°C / 71 .6°F) is not achieved with 4 minutes of start-up.

Action Taken When the DTC Sets

- The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic test has been run and failed.
- The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC

- The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
- The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
- The DTC can be cleared by using the Scan Tool.

Diagnostic Aids

The DTC P0125 diagnostic test will not run if the engine is started while the coolant temperature is less than 10°C (50°F) or greater than 22°C (71.6°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 indicates a slewed 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, replace the ECT sensor.
- Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
- Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

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:

2. 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 fault is intermittent; refer to Diagnostic Aids.