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 powertrain control module (PCM) controls fuel delivery in open loop, ignoring the heated oxygen sensor (HO2S) 1 signal and calculating the air/fuel ratio based upon inputs from the engine coolant temperature (ECT), throttle position (TP), and mass air flow (MAF) sensors. The PCM begins using the HO2S 1 signal for controlling fuel delivery (Closed Loop) when the following conditions are met:
| • | The HO2S 1 signal is varying, indicating that the sensor is hot enough to operate properly. |
| • | The ECT sensor indicates a coolant temperature of more than 20°C (68°F). |
| • | The 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 intake air temperature (IAT) or ECT sensor DTCs are present |
| • | The IAT is more than -40°C (-40°F). |
| • | The ECT is more than -40°C (-40°F). |
Conditions for Setting the DTC
The engine coolant takes more than 2 and 5 minutes to reach a stabilized temperature of 20°C (68°F). The proper amount of time depends upon the engine coolant temperature at startup and the amount of air cycled by the engine since startup.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has run and failed. |
| • | The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
The DTC P0125 diagnostic test does 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 the vehicle cannot be operated under the appropriate conditions, test or inspect the following items:
| • | The cooling system coolant level. |
| • | The thermostat operation. |
| • | The cooling fans' operation. |
| • | Test for high resistance in the wiring related to the ECT sensor. Also, inspect for faulty connections at the ECT sensor and the 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 the actual coolant temperature as 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. |
| • | A faulty connection at PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections. |
| • | Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display indicates the location of the malfunction. |
If DTC P0125 cannot be duplicated, the information included in the Fail Records data may be useful in determining vehicle mileage since the DTC was last set.
Test Description
The number(s) below refer to the step number(s) on the Diagnostic Table:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Does the temperature exceed the specified value within 2 minutes? | 21°C (70°F) | Go to Diagnostic Aids | |
3 | Are any of the following engine4 coolant temperature sensor (ECT) sensor DTCs set? | -- | Go to the other DTC first | |
4 |
Are the cooling fans off? | -- | ||
5 | Inspect the cooling system coolant level. Is the coolant level OK? | -- | ||
6 | Test for proper operation of the thermostat. Refer to Thermostat Diagnosis in Engine Cooling. Is the thermostat operating correctly? | -- | ||
Compare the ECT displayed on the scan tool to the actual coolant temperature measured with a thermometer. Is the scan tool ECT within the specified value of the actual temperature? | ±3°C (±5°F) | |||
8 |
Was a problem found? | -- | ||
9 | Repair the condition as necessary. Refer to Engine Cooling. Did you complete the repair? | -- | -- | |
10 | Replace the loose terminal(s) or repair the faulty wiring as necessary. Refer to Wiring Repairs or to Connector Repairs in Wiring Systems.. Did you complete the repair? | -- | -- | |
11 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the repair? | -- | -- | |
12 |
Does the ECT exceed the specified value within 2 minutes? | 21°C (70°F) | System OK |
