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 fuel trim values more than 23 percent for long term fuel trim or more than 25 percent for short term fuel trim.
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
| • | 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. |
| • | If this DTC cannot be duplicated, the DTC may have been caused by the customer running the vehicle out of fuel. |
| • | If the opposite bank DTC is also set, this indicates that both banks of the engine are operating lean. Inspect all areas of the engine for a vacuum leak. Also inspect the positive crankcase ventilation (PCV) system for leaks. Make sure that the engine oil fill cap is tight. Check that the engine oil dip stick is fully seated. |
| • | Fuel delivery--A fuel delivery malfunction can cause this DTC to set. Thoroughly inspect all items that cause a lean condition. |
| • | Fuel contamination--Water, even in small amounts, near the in-tank fuel pump inlet, can be delivered to the injectors. The water causes a lean condition and can set these DTCs. |
| • | Secondary air injection (AIR) system--Inspect the AIR system for proper operation and verify that all the components are connected. |
| • | A skewed mass air flow (MAF) sensor may be the cause of these DTCs. |
| • | Intermittents--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 lean.
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This step addresses areas of concern that could cause both banks of the engine to run lean.
Step | Action | Values | Yes | No | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||||||||
2 | Is DTC P1460 set? | -- | Go to DTC P1460 | Go to Step 3 | ||||||||||||||
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Important: If any DTCs are set, except P0171 and P0174, refer to those DTCs before proceeding with this diagnostic.
Is the LT FT less than the specified value? | +23 % LT | Go to Step 4 | Go to Step 5 | |||||||||||||||
Did the DTC fail this ignition? | -- | Go to Step 5 | Go to Diagnostic Aids | |||||||||||||||
Are both DTCs P0171 and P0174 set? | -- | Go to Step 6 | Go to Step 7 | |||||||||||||||
Visually/physically inspect the following items:
Did you find and correct the condition? | -- | Go to Step 9 | Go to Fuel System Diagnosis | |||||||||||||||
7 | Visually/physically inspect the following items:
Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 8 | ||||||||||||||
8 | Test for a malfunctioning oxygen sensor or circuit. Refer to DTC P0132 or P0152 . Did you find and correct the condition? | -- | Go to Step 9 | Go to Fuel System Diagnosis | ||||||||||||||
9 |
Does the DTC run and pass? | -- | Go to Step 10 | Go to Step 2 | ||||||||||||||
10 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||||||||||
