Circuit Description
To provide the best possible combination of driveability, fuel economy and emission control, a Closed Loop air/fuel metering system is used. While in a Closed Loop operation, 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 (Additive and Multiplicative) fuel trim diagnostic parameter is a calculated value from several of the long term speed load learn cells which are selected based on speed and load.
Conditions for Running the DTC
| • | A closed loop operation |
| • | Fuel trim enabled |
Conditions for Setting the DTC
The ECM detects fuel trim values greater than 550 milliseconds for additive fuel trim or greater than 23 percent for multiplicative 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. |
| • | The first drive trip that this diagnostic fails (Fail Pending), this 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 failure 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 does not fail within the same conditions that the DTC last failed. |
| 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. |
| • | Fuel pressure: The system will be lean if the pressure is too low. It may be necessary to monitor the fuel pressure while driving the vehicle at various road speeds and/or loads in order to confirm. |
| • | Fuel contamination: Water, in even small amounts, near the in-tank fuel pump inlet, can be delivered to the injector. The water causes a lean exhaust and can set DTC P0171. |
| • | Refer to Symptoms for the Intermittents. |
Test Description
The number(s) below refer to the step numbers on the diagnostic table:
-
Determines whether the fault is present.
-
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 within 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.
-
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.
-
Inspect all areas of the engine for a vacuum leak. Also inspect the PCV system for leaks. Make sure that the engine oil fill cap is in place and tight. Check that the engine oil dip stick is fully seated.
Step | Action | Value(s) | Yes | No | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System check? | -- | ||||||||||||||||
|
Important: If any DTCs are set (Except P0171 and P0174), refer to those DTCs before proceeding with this diagnostic.
Are both parameters less than the specified value? | 95% | |||||||||||||||||
Is this DTC set? | -- | Refer to Diagnostic Aids | ||||||||||||||||
Is DTC P0174 also set? | -- | |||||||||||||||||
Visually/physically inspect the following items:
Was a problem found and corrected? | -- | Refer to Fuel System Diagnosis | ||||||||||||||||
6 | Visually/physically inspect the following items:
Was a problem found and corrected? | -- | Refer to Fuel System Diagnosis | |||||||||||||||
7 |
Is this DTC set? | -- | ||||||||||||||||
8 | Were there any additional DTCs noted during the Powertrain OBD System Check that have not been diagnosed? | -- | Go to the applicable DTC table | The System is OK |
