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° (70°F). |
| • | Time since startup 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 40°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 150 seconds to achieve closed loop temperature of 20°C (70°F). |
| • | Calibrated minimum amount of total air flow has been exceeded. |
| • | Maximum idle time less than 125 seconds. |
Region 2:
| • | Engine run time over 250 seconds to achieve closed loop temperature of 20°C (70°F). |
| • | Calibrated minimum amount of total air flow has been exceeded. |
| • | Maximum idle time less than 200 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 or by disconnecting the PCM battery feed. |
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:
- 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 Intermittents and Poor Connections Diagnosis.
- 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 .
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:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 |
Does Eng. Cool. Temp. increase to above the specified value within 2 minutes? | 20°C (70°F) | Go to Diagnostic Aids | |
3 | Are any of the following ECT sensor DTCs set? | -- | Go to applicable DTC | |
4 |
Are the cooling fans off? | -- | ||
5 | Check the cooling system coolant level. Refer to Thermostat Diagnosis in Engine Cooling. Is the coolant level OK? | -- | ||
6 | Check for proper operation of the thermostat . Refer to Thermostat Diagnosis in Engine Cooling. Is the thermostat operating correctly? | -- | ||
Compare Engine Coolant Temperature displayed on the scan tool to the actual coolant temperature measured with a thermometer. Is the scan tool Engine Coolant Temperature indication within the specified value of the measured temperature? | ± 3°C (± 5°F) | |||
8 |
Was a problem found? | -- | ||
9 | Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling for cooling system diagnosis and repair condition as necessary. Is action complete? | -- | -- | |
10 | Replace the loose terminal(s) or repair malfunctioning wiring as necessary. Refer to Wiring Repairs . Is action complete? | -- | -- | |
11 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is action complete? | -- | -- | |
12 |
Does Eng. Cool. Temp. increase to above the specified value within 2 minutes? | 20°C (70°F) | System OK |
