P0171

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 closed loop, the PCM monitors the H025 1 signal and adjusts fuel delivery based upon the H025 signal voltages. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0%; if the H025 signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, 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 PCMs maximum authority to control long term fuel trim allows a range between -29% and +22%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells are as follows:

The vehicle may have to be operated in all of the above conditions marked by an X, before the PCM determines fuel trim status. The amount of fuel trim deviates from 0% 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 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.
^ Engine coolant temperature between 20°C (68°F) and 110°C (230°F).
^ Intake air temperature is between -18°C (0°F) and 70°C (158°F).
^ Manifold absolute pressure is between 15 kPa and 85 kPa.
^ Throttle angle is steady and less than 90%.
^ Vehicle speed is below 132 km/h (82 mph).
^ Engine speed is between 600 and 4000 RPM.
^ Baro is greater than 70 kPa.
^ Air flow is between 3 gm/s and 150 gm/s.

Conditions for Setting the DTC
^ Long term fuel trim is at or near maximum authority of 22%.
^ Short term fuel trim is greater than 27%.

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 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 the MIL OFF 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 the scan tool.

Diagnostic Aids
Check for the following conditions:
^ Heated Oxygen Sensor wiring - The sensor pigtail may be routed incorrectly and contacting the exhaust system.
^ Poor PCM to engine block grounds. Refer to Power and Ground Component Views in Diagrams.
^ Poor connection at the PCM - Inspect harness connectors for the following conditions:
- Backed out terminals.
- Improper mating.
- Broken locks.
- Improperly formed or damaged terminals.
- Poor terminal to wire connection.
^ Fuel pressure low - The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0171 may be set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Fuel injector(s) faulty - Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
^ Vacuum leaks - Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system. Refer to Visual/Physical inspection in Symptoms. Symptom Related Diagnostic Procedures
^ Exhaust leaks - An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the H025, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Inspection Procedure.
^ Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor. Refer to MAF Sensor Replacement
^ Fuel contamination - Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
^ Poor connection at PCM - Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection(s). Refer to Intermittents and Poor Connections.
^ Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the fault.

Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.

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