Circuit Description
While in Closed Loop, the engine control module (ECM) monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage. A change made to fuel delivery will be indicated by the Long and Short Term Fuel Trim values. The Short Term Fuel Trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The Long Term Fuel Trim values change in response to trends in the Short Term Fuel Trim. The Long Term Fuel Trim makes coarse adjustments to the fueling in order to re-center and restore control to the Short Term Fuel Trim. The Short and the Long Term Fuel Trim can be monitored with a scan tool. Ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. If an excessively rich or lean condition is detected, the ECM will set a diagnostic trouble code (DTC). The Long Term Fuel Trim diagnostic parameter is an average of several of the long term speed load learn cells which the ECM selects based on the engine speed and the engine load.
Conditions for Running the DTC
| • | Closed Loop operation |
| • | Fuel trim enabled |
Conditions for Setting the DTC
The ECM detects the average Long Term Fuel Trim cell values are below a predetermined threshold.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Diagnostic Aids
In terms of fuel trim, a value less than -8.5 percent would be -9.0 percent. This value indicates that the ECM is leaning the fuel mixture, or reducing the fuel in order to compensate for a rich condition. A positive value indicates the ECM is richening the fuel mixture, or adding more fuel in order to compensate for a lean condition. If using the diagnostic table does not isolate the problem, try monitoring the Long Term Fuel Trim cell while operating the vehicle under various loads. Monitor the engine data list for the possibility of a skewed sensor causing a driveability concern. This may isolate the condition which caused the setting of the current DTC. IF DTCs P0172 and P0175 are both set, inspect the fuel pressure regulators atmospheric vent hose for signs of fuel. The vent hose connects to the bottom side of the throttle body. If the fuel pressure regulator has failed the engine will burn the excess fuel causing a rich condition. A fault in the EVAP system could also cause a rich condition. Inspect the evaporative emission (EVAP) system for proper operation.
Use the J 35616 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector. Using this kit prevents damage to the harness or component terminals. Refer to Using Connector Test Adapters in Wiring Systems. For an intermittent, refer to Symptoms - Engine Controls . If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines if the fault is present.
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This process may help in order to recreate the malfunction.
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This step determines if both banks are running rich.
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This step addresses areas of concern that could cause both banks of the engine to run rich.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||||
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Important: If any DTCs are set, except P0172 and P0175, refer to those DTCs before proceeding with this diagnostic.
Is the Long Term Fuel Trim less than the specified value? | -8.5% LT | Go to Step 4 | Go to Step 3 | |||||||||||
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids | |||||||||||
Are both DTCs P0172 and P0175 set? | -- | Go to Step 5 | Go to Step 6 | |||||||||||
Visually/physically inspect the following items:
Did you find and correct the condition? | -- | Go to Step 8 | Go to Fuel System Diagnosis | |||||||||||
6 |
Important: When the fuel system check is finished, return to the beginning of this table. Inspect the Bank 1 injectors for leaks. Refer to Fuel System Diagnosis . Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 7 | ||||||||||
7 | Test for a malfunctioning oxygen sensor or circuit. Refer to DTC P0131 or P0151 . Did you find and correct the condition? | -- | Go to Step 8 | Go to Fuel System Diagnosis | ||||||||||
8 |
Does the DTC run and pass? | -- | Go to Step 9 | Go to Step 2 | ||||||||||
9 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||||||
