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 startup is at least 10 seconds for a warm engine or 50 seconds for a cold engine. |
Conditions for Running the DTC
| • | No IAT or ECT sensor DTC(s) present. |
| • | Intake air temperature is more than -7°C (19°F). |
| • | Engine coolant temperature is more than -40°C (-40°F). |
Conditions for Setting the DTC
The length of time for the engine coolant to reach a stabilized temperature of 20°C (68°F) takes longer than it should. The amount of time ranges between 2 and 5 minutes depending on engine coolant temperature at startup and the amount of air cycled by the engine since startup.
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 a scan tool. |
Diagnostic Aids
The DTC P0125 diagnostic test will not run if the engine is started while the coolant temperature is less than 0°C (32°F) or more than 21°C (70°F). If it is not possible to operate the vehicle under the appropriate conditions, check the following items:
| • | Cooling system coolant level. Refer to Engine Cooling System Specifications in Engine Cooling. |
| • | Thermostat operation. Refer to Thermostat Diagnosis in Engine Cooling. |
| • | Cooling fans operation. Refer to Electric Cooling Fan Diagnosis . |
| • | Check for high resistance in wiring related to the ECT sensor. Also, check for poor connections at ECT sensor and PCM. Refer to Wiring Repairs in Wiring Systems. |
| • | 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 . |
| Refer to Temperature Versus Resistance . |
| • | 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 ECT 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. |
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:
-
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 .
If the temperatures are close, the malfunction is intermittent; refer to Diagnostic Aids.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Does Eng. Cool. Temp. increase to above the specified value within 2 minutes? | 21°C (70°F) | Go to Diagnostic Aids | |
3 | Are any of the following ECT sensor DTCs set? | -- | Go to the other DTC first | |
4 |
Are the cooling fans off? | -- | ||
5 | Check the cooling system coolant level. Refer to Engine Cooling System Specifications 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 Coolant Thermostat Replacement in Engine Cooling for repair condition as necessary. Did you complete the repair? | -- | -- | |
10 | Replace the loose terminals or repair malfunctioning wiring as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
11 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the repair? | -- | -- | |
12 |
Does Eng. Cool. Temp. increase to above the specified value within 2 minutes? | 21°C (70°F) | System OK |
