P0171

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Fuel Trim Cells:

CIRCUIT DESCRIPTION
A closed loop air/fuel metering system is used in order to provide the best possible combination of driveability, fuel economy, and emissions control. While in closed loop, the PCM monitors the HO2S 1 signal and adjusts the fuel delivery based on the HO2S signal voltages. A change to the fuel delivery will be indicated by the long and short term fuel trim values, which can be monitored with a scan tool. The ideal fuel trim values are approximately 0 percent. If the HO2S signals indicate a lean condition the PCM will add fuel, resulting in fuel trim values that are more than 0 percent. If a rich condition is detected, the fuel trim values will be less than 0 percent, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, the PCM will set DTC P0171. The PCM's maximum authority to control long term fuel trim allows a range from 30-+22 percent. The PCM's maximum authority to control the short term fuel trim allows a range from -27-+27 percent. The PCM monitors the fuel trim during various operating and fuel trim cell conditions before determining the status of the fuel trim diagnostic. The fuel trim cells are shown in the table.

The vehicle may have to be operated in all of the above conditions marked by an X before the PCM determines the fuel trim status. The amount of fuel trim deviates from 0 percent in each cell, and the amount that each cell is used directly affects whether or not the vehicle must be operated in all of the cells described above in order to complete the diagnostic.

CONDITIONS FOR RUNNING THE DTC
^ No MAF, MAP, TP, ECT, IAT, CKP, CAM sensor, Misfire, Fuel Injector, VSS, EST, HO2S, EGR, or EVAP DTCs are set.
^ The engine coolant temperature is between 20°C (68°F) and 110°C (230°F).
^ The intake air temperature is between -18°C (0°F) and 70°C (158°F).
^ The manifold absolute pressure is between 15 kPa and 80 kPa.
^ The throttle angle is steady and less than 90 percent.
^ The vehicle speed is less than 132 km/h (82 mph).
^ The engine speed is between 600-4000 RPM.
^ The baro is more than 70 kPa.
^ The air flow is between 3 gm/s and 150 gm/s.

CONDITIONS FOR SETTING THE DTC
^ The long term FT average is more than 20 percent.
^ The short term FT is more than +3 percent.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) during the second consecutive trip in which the diagnostic has been run and failed.
^ If equipped with traction control, the PCM will command the Electronic Brake And Traction Control Module (EBTCM) via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been 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
Use the table to identify the system or component that is suspected:

Also inspect for:

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.

^ Poor terminal connection - Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension.
^ 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.
^ PCM and engine grounds for clean and secure connections

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
2. DTCs other than P0171 may indicate the presence of a condition which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct DTC P0171.
4. If the DTC P0171 test passes while the Failure Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids or Symptoms for additional information on diagnosing intermittent problems.