Code P0171, Bank 1 Fuel Trim System Too 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.

Engine fuel system is operating in closed loop.
Fuel trim is enabled.
BARO is greater than 10.2 psi (70 kPa).
TP sensor angle is less than 70 percent.
Engine RPM is 500-4000 RPM.
MAP is 2.9-13.1 psi (20-90 kPa).
IAT is -22°F (-30°C) and 140°F (60°C).
ECT is 140°F (60°C) and 239°F (115°C).
MAF is 5 grams per second and 100 grams per second.
Vehicle speed is less than 75 MPH.
EVAP canister purge duty cycle is less than 75 percent.
Codes P0114, P0115, P0116, P0117, P0118, P0133, P0134, P0153 and P0154 are not set.
All conditions are met for 10 seconds.
Average short term fuel trim is 138.
Average long term fuel trim is 155.

Code P0171 will be stored in PCM memory, but will not illuminate Malfunction Indicator Light (MIL). A failed last test code will clear when the fault conditions no longer exist. A history code will clear after 40 ignition cycles without a fault, when the battery has been disconnected for 30 seconds, or when the code clearing feature on the Tech 1 is used.

NOTE: Test numbers refer to numbers on diagnostic chart.

Codes P0171 and P0174 set at the same time indicates both banks are operating lean. Inspect components/systems that could cause this condition (MAF, vacuum leak, fuel contamination, etc.).
If bank 1 is operating lean, short term fuel trim will be near 138, and long term fuel trim will be near 155. These parameters are an average of all fuel trim cells.
If fuel pressure is too low, system will go lean. Codes P0171 and P0174 may set if fuel pressure is too low.
A lean condition will be caused if the air pump is operating in closed loop.
MAF sensor could cause a lean condition, but not set a MAF code. If lean condition no longer exists when MAF sensor is disconnected, replace MAF sensor. Visually/physically inspect both sensors for contamination. If contamination is evident, find cause of contamination (lead, silicone, heavy engine oil/coolant consumption, etc.).