Circuit Description
While in Closed Loop, the ECM monitors the heated oxygen sensor 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 DTC. The long term fuel trim diagnostic parameter is a value calculated on the basis of speed and 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 ECM will illuminate the malfunction indicator lamp (MIL) on the second consecutive drive trip that the diagnostic runs and fails. If the diagnostic fails under the same conditions (the load, the RPM, the temperature, etc.) as the previous ignition cycle that the test ran and failed. |
| • | On the first drive trip that this diagnostic fails (Fail Pending), the information will be stored in the Freeze Frame. Any data that was previously stored in the Freeze Frame will be overwritten by the Freeze Frame data for this DTC. The only exception to this is if a Misfire DTC was already recorded in the Freeze Frame. In this case, the Misfire data stays in the Freeze Frame and the Fuel Trim data is not stored. |
Conditions for Clearing the MIL/DTC
| Important: If the last fault was during a non-typical driving condition, the MIL may remain on longer than three drive trips. Review the Freeze Frame for the last failure conditions. |
| • | The ECM turns off the MIL after three consecutive drive trips that the diagnostic runs and passes within the same conditions that the DTC last set. |
| A History DTC clears after forty consecutive warm-up cycles if no failures report by this diagnostic or any other emission related diagnostic. |
| • | The ECM battery voltage is interrupted. |
| • | The scan tool clears the MIL/DTC. |
Diagnostic Aids
| • | If a DTC P0171 cannot be duplicated, the DTC may have been caused by the customer running the vehicle out of fuel. |
| • | If DTC P0174 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 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. A skewed mass air flow (MAF) sensor may be the cause of these DTCs. |
| • | 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 a DTC P0171. |
| • | Secondary AIR System--Inspect the AIR system for proper operation and verify that all the components are connected. |
| • | Intermittents--For an intermittent, refer to Symptoms . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines whether the fault is present.
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Using the Freeze Frame data may aid in locating an intermittent condition. If the DTC cannot be duplicated, review the information in the Freeze Frame. Try to operate the vehicle with in the same Freeze Frame conditions, the RPM, the MAF, the vehicle speed, the temperature, etc., that were noted. This process may help in order to recreate the malfunction.
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If DTC P0174 is also set, this indicates that both banks of the engine are operating lean. Inspect all items that would cause both banks to operate lean.
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Inspect all areas of the engine for a vacuum leak. Also inspect thepositive 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.
Step | Action | Values | Yes | No | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System check? | -- | Go to Step 2 | |||||||||||||||
2 | Is DTC P1460 set? | -- | 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 either parameters more than the specified value? | +8.5% LT +23% ST | Go to Step 5 | Go to Step 4 | |||||||||||||||
Is this DTC set? | -- | Go to Step 5 | Go to Diagnostic Aids | |||||||||||||||
Is DTC P0174 also set? | -- | Go to Step 6 | Go to Step 7 | |||||||||||||||
Perform the following visual/physical inspections and tests:
Did you find and correct a problem? | -- | Go to Step 9 | Go to Fuel System Diagnosis | |||||||||||||||
7 | Perform the following visual/physical inspections and tests:
Did you find and correct a problem? | -- | Go to Step 9 | Go to Step 8 | ||||||||||||||
8 | Test for a malfunctioning oxygen sensor or circuit. Go to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 . Did you find and correct a problem? | -- | Go to Step 9 | Go to Fuel System Diagnosis | ||||||||||||||
9 |
Does the scan tool indicate that the diagnostic Passed? | -- | Go to Step 10 | Go to Step 2 | ||||||||||||||
10 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
