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 10°C (50°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 DTCs present |
| • | Intake air temperature is greater than -7°C (19°F). |
| • | Engine coolant temperature is greater than -40°C (-40°F). |
| • | Start up coolant temperature less than 50°C (122°F). |
Conditions for Setting the DTC
An excessive amount of time passes before engine coolant temperature reaches the Closed Loop operation (10°C /50°F) threshold. The amount of time ranges between 2 and 13 minutes depending on engine coolant temperature at startup and the amount of air cycled through the engine since startup.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL Off after three consecutive drive trips that the diagnostic runs and does not fail. |
| • | A last test failed (current DTC) clears when the diagnostic runs and does not fail. |
| • | A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures. |
| • | Use a scan tool in order to clear the MIL/DTC. |
| • | Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module (PCM) Description , Clearing Diagnostic Trouble Codes. |
Diagnostic Aids
Notice: Use the connector test adapter kit J 35616-A for any test that
requires probing the following items:
• The PCM harness connectors • The electrical center fuse/relay cavities • The component terminals • The component harness connector
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 greater 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. |
| • | 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 Intermittents and Poor Connections Diagnosis in Wiring Systems. |
| • | DTC P0125 set may indicate 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, and the ECT circuit/connections check out OK, replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . |
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.
If the problem is intermittent, refer to Intermittent Conditions .
Test Description
An example test description follows:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Important:: Time to Closed Loop can not be tested if start-up coolant temp is 21°C (70°F) or greater. If it is not possible to cool the engine below this temperature do not use this table. Refer to Diagnostic Aids.
Does ECT increase to above the specified value within 2 minutes? | 21°C (70°F) | Refer to Diagnostic Aids. | |
3 | Is DTC P0117, P0118, P0480 or P0481 set? | -- | Diagnose these DTCs first | |
4 |
Are the cooling fans off? | -- | ||
5 | Check the cooling system coolant level. . Is the coolant level OK? | -- | ||
6 | Check for proper operation of the thermostat . Refer to Engine Fails To Reach Normal Operating Temperature 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 Engine Cooling for cooling system diagnosis and repair condition as necessary. Is the action complete? | -- | -- | |
10 | Replace the loose terminal(s) or repair malfunctioning wiring as necessary. Refer to Intermittents and Poor Connections Diagnosis in Electrical Diagnosis. Is the action complete? | -- | -- | |
11 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | -- | |
12 |
Does ECT increase to above the specified value within 2 minutes? | 21°C (70°F) | ||
13 | Review Captured Info using the scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
