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 HO2S signals and calculating the air/fuel ratio based on inputs from the engine coolant temperature, the throttle position, and the mass air flow sensors. The PCM will begin using the HO2S 1 signal for controlling fuel delivery 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 that the coolant temperature is more than 15°C (64°F). |
| • | The 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 3 temperature regions. The diagnostic uses the ECT and IAT sensors in order 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:
| • | The total airflow must exceed a calibrated amount. |
| • | The engine idle time must be less than a calibrated amount. |
Conditions for Running the DTC
| • | No active IAT or ECT Sensor DTCs are present. |
| • | The minimum air temperature is more than -7°C (+19°F). |
| • | The start-up engine coolant temperature is less than 50°C (122°F). |
| • | Region 1--air temperature is above 10°C (50°F) and the start- up coolant temperature is more than 10°C (50°F) . |
| • | Region 2--air temperature is more than -7°C (+20°F) and the start- up coolant temperature is between -7°C (+20°F) and 10°C (50°F). |
| • | Region 3--air temperature is more than -7°C (+20°F) and the start- up coolant temperature is between -40°C (-40°F) and -7°C (20°F). |
Conditions for Setting the DTC
Region 1:
| • | The engine run time is more than 115 seconds in order to achieve Closed Loop temperature of 15°C (64°F). |
| • | The calibrated minimum amount of total airflow has been exceeded. |
| • | The maximum idle time is less than 90 seconds. |
Region 2:
| • | The engine run time is more than 268 seconds in order to achieve Closed Loop temperature of 15°C (64°F). |
| • | The calibrated minimum amount of total airflow has been exceeded. |
| • | The maximum idle time is less than 195 seconds. |
Region 3:
| • | The engine run time is more than 390 seconds in order to achieve Closed Loop temperature of 15°C (64°F). |
| • | The calibrated minimum amount of total airflow has been exceeded. |
| • | The maximum idle time is less than 300 seconds. |
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 the 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 .
Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Loose terminal connection |
| - | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| - | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| • | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
| • | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.
Test Description
The numbers below refer to the step numbers on the diagnostic table:
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||
2 | Are any ECT sensor DTCs set? | -- | Go to applicable DTC | |
3 |
Is the ECT parameter near the specified value? | 140°C (284°F) | ||
4 | Inspect for high resistance in the wiring related to the ECT sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
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 replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | -- | |
8 |
Does ECT increase to above the specified value within 9 minutes? | 15°C (64°F) | System OK |
