Code P0171: Fuel Trim Lean: Notes
A closed loop air/fuel metering system is used to provide best possible combination of driveability, fuel economy and emission control. While in closed loop, PCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. Scan tester monitors fuel trim values, and will indicate a change made to fuel delivery by long and short term fuel trim values.
Ideal fuel trim values are about 128. If oxygen sensor signal indicates a lean condition, PCM will add fuel, resulting in a fuel trim value greater than 128. If a rich condition is detected, fuel trim values will be less than 128, indicating that PCM is reducing amount of fuel delivered. PCM will set Code P0171 if an excessively lean condition is detected. Code P0171 will set when the following conditions are present.
- Codes P0101, P0117, P0118, P0122, P0123, P0134, P0501 and P0502 are not set.
- Engine coolant temperature is between 176°F (80°C) and 230°F (110°C).
- Steady throttle less than 91 percent.
- Vehicle speed is less than 70 MPH.
- On VIN L, engine RPM is between 1000-4000 RPM, or on VIN 1, engine RPM is 0-4000 RPM.
- EVAP canister purge duty cycle is less than 80 percent.
- Long term fuel trim is greater than 158.
- Short term fuel trim is 180.
- All conditions exist for more than 25 seconds.
Preliminary Inspection
- Check exhaust for corrosion, loose or missing components.
- Ensure oxygen sensor is installed properly and pigtail is not contacting hot exhaust or ignition wires.
- Check vacuum hoses for splits, kinks and proper routing and connections.
- Check for vacuum leaks at throttle body, intake manifold and EGR valve.
- Check for proper operation of IAC system.
- Check proper operation of PCV system.
- Check throttle body inlet screen for damage and/or foreign objects.
- Check fuel for excessive water, alcohol, or other contaminates.
- Ensure PCM and sensor grounds are clean and tight.NOTE: To complete a trip cycle for fuel trim diagnostic, fuel trim diagnostic test must be enabled and run according to conditions required to set Code P0171. A trip is not considered complete until fuel trim diagnostics have been run by PCM.NOTE: Test numbers refer to numbers on diagnostic chart.
- Visually/physically checking components/systems which may cause a lean condition may determine cause of Code P0171.
- MAF sensor which causes PCM to read a lower than actual airflow can cause a lean condition, and Code P0171 to set.
- Check for faults in EVAP purge system that may cause Code P0171 to set.