Code P0171, Fuel Trim Lean: Notes

A closed loop air/fuel metering system is used to provide the 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. The Tech 1 monitors fuel trim values, and will indicate a change made to fuel delivery by the long and short term fuel trim values.

Ideal fuel trim values are approximately 128. If oxygen sensor signal indicates a lean condition, the 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 the PCM is reducing the 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.
Above conditions for more than 25 seconds.

When Code P0171 sets, a software flag indicating an emissions related fault will set for 3 trips, after which flag will clear, provided lean condition no longer exists. PCM will illuminate Malfunction Indicator Light (MIL) if conditions necessary for Code P0171 are detected during a trip while flag is set. MIL will remain illuminated for all subsequent trips until code has been cleared or lean condition has not been detected, and conditions for checking fuel trim have been met for 3 consecutive trips.

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 diagnostic has 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 then 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.