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 HO2S signals 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 output signal is varying, indicating that the sensor is hot enough to operate properly. |
| • | The engine coolant temperature sensor indicates coolant temperature above 21°C (70°F). |
| • | 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 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 more than -7°C (19°F). |
| • | Start-up engine coolant temperature is below 50°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 127 seconds to achieve closed loop temperature of 15°C (64°F). |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 95 seconds. |
Region 2:
| • | Engine run time over 280 seconds to achieve closed loop temperature of 15°C (64°F). |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 210 seconds. |
Region 3:
| • | Engine run time over 439 seconds to achieve closed loop temperature of 15°C 640°F). |
| • | Calibrated minimum amount of total airflow has been exceeded. |
| • | Maximum idle time less than 329 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 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 .
Inspect for the following conditions:
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 the applicable DTC table | |
3 |
Is the ECT parameter near the specified value? | 140°C (284°F) | ||
4 |
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 new PCM must be reprogrammed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the repair? | -- | -- | |
8 |
Does ECT increase to above the specified value within 9 minutes? | -10°C (14°F) | System OK |
