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) VIN K OR 20°C (68°F) VIN1. |
| • | 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) VIN K OR 10°C (50°F) VIN 1. |
| • | Region 2--air temperature above -7°C (20°F) and the start up coolant temperature between -7°C (20°F)-10°C (50°F). |
| • | Region 3--air temperature above -7°C (20°F) and the start up coolant temperature between -40°C (-40°F) to -7°C (20°F). |
Conditions for Setting the DTC
Region 1:
| • | Engine run time over 6 seconds VIN K OR 150 seconds VIN 1 to achieve closed loop temperature of -10°C (14°F) VIN K OR 20°C (68°F) VIN 1. |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 5 seconds VIN K OR 125 seconds VIN 1. |
Region 2:
| • | Engine run time over 154 seconds VIN K OR 250 seconds VIN 1 to achieve closed loop temperature of -10°C (14°F) VIN K OR 20°C (68°F) VIN 1. |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 120 seconds VIN K OR 200 seconds VIN 1. |
Region 3:
| • | Engine run time over 287 seconds VIN K or 375 seconds VIN 1 to achieve closed loop temperature of -10°C (14°F) VIN K or 20°C (68°F) VIN 1. |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 225 seconds VIN K or 300 seconds VIN 1. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (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/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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. Refer to Engine Coolant Temperature Sensor Replacement .
Check for the following conditions:
| • | 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. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
| • | 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. Refer to Wiring Repairs in Wiring Systems. |
If DTC P0125 cannot be duplicated, the information included in the Failure Records data can be useful in determining vehicle mileage since the DTC was last set.
Test Description
The numbers below refer to the Step numbers on the diagnostic table:
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
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 new PCM must be reprogrammed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the repair? | -- | -- | |
8 |
Does DTC P0125 run and pass? | -- | System OK |
