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 HO2S signals 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 output signal is varying, indicating that the sensor is hot enough to operate properly.
^ The Engine Coolant Temperature sensor indicates coolant temperature above 21°C (70°F).
^ Time since start-up is at least 15 seconds for a warm engine or 90 seconds for a cold engine.

The PCM monitors the amount of time for the cooling system to achieve closed loop operating temperature. This diagnostic is broken into three temperature regions. The diagnostic uses the ECT and IAT Sensors to determine which region to operate in. The time to achieve closed loop temperature is monitored. A time based decision is made depending on the temperature region. The diagnostic will monitor the following conditions prior to setting the DTC:

^ Total airflow must exceed a calibrated amount.
^ Engine idle time must be less than a calibrated amount.

Conditions for Running the DTC

^ No active IAT or ECT Sensor DTCs present.
^ Minimum air temperature is greater than -7°C (19°F).
^ Start-up engine coolant temperature is below 50°C (122°F).
^ Region 1-air temperature above 10°C (50°F) and the start up coolant temperature above 10°C (50°F).
^ Region 2-air temperature above -7°C (20°F) and the start up coolant temperature between -7°C (20°F) and 10°C (50°F).
^ Region 3-air temperature above -7°C (20°F) and the start up coolant temperature between -40°C (-40°F) and -7°C (20°F).

Conditions for Setting the DTC

Region 1:
^ Engine run time over 127 seconds to achieve closed loop temperature of 21°C (70°F).
^ Calibrated minimum amount of total airflow has been exceeded.
^ Maximum idle time less than 95 seconds.

Region 2:
^ Engine run time over 280 seconds to achieve closed loop temperature of 21°C (70°F).
^ Calibrated minimum amount of total airflow has been exceeded.
^ Maximum idle time less than 210 seconds.

Region 3:
^ Engine run time over 439 seconds to achieve closed loop temperature of 21°C (70°F).
^ Calibrated minimum amount of total airflow has been exceeded.
^ Maximum idle time less than 329 seconds.

Action Taken When the DTC Sets

^ The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed.
^ If equipped with traction control, the PCM will command the Electronic Brake and Traction Control Module (EBTCM) via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
^ 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 a scan tool.

Diagnostic Aids

DTC P0125 set indicates 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. replace the ECT sensor.

Check for the following conditions:

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating. broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.

Test Description

The number below refers to the step number on the Diagnostic Table:

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.