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 -10°C (+14°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 DTCs present. |
| • | Minimum air temperature is more than -7°C (+19°F). |
| • | Start-up engine coolant temperature is below 40°C (104°). |
| • | Region 1--air temperature above 10°C (50°F) and the start up coolant temperature above -10°C (+14°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 6 seconds to achieve closed loop temperature of -10°C (+14°F). |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 5 seconds. |
Region 2
| • | Engine run time over 154 seconds to achieve closed loop temperature of -10°C (+14°F). |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 120 seconds. |
Region 3
| • | Engine run time over 287 seconds to achieve closed loop temperature of -10°C (+14°F). |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 225 seconds. |
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 Clear Info function. |
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. |
| • | Inspect for the following conditions: |
| - | Poor connection at PCM |
| - | Backed out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal to wire connection--Use a corresponding mating terminal to check for proper terminal tension. Refer to Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
| - | Damaged harness--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 malfunction. Refer to Wiring Repairs . |
| • | 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
The number below refers to the step numbers on the diagnostic table:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain ON-Board Diagnostic System Check? | -- | ||
2 | Are any ECT sensor DTCs set? | -- | Go to applicable DTC. Refer to Diagnostic Trouble Code (DTC) List/Type . | |
3 |
Is the ECT parameter near the specified value? | 140°C or 284°F | ||
4 |
Was a problem found? | -- | ||
5 | Replace the faulty terminals or repair the faulty wiring as necessary. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
6 | Replace ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | -- | |
7 |
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the repair? | -- | -- | |
8 |
Does DTC P0125 run and pass? | -- | System OK |
