P0171

Code P0171, Fuel Trim Lean (Part 1 Of 2):

Code P0171, Fuel Trim Lean (Part 2 Of 2):

Heated Oxygen Sensor (HO2S) Circuit:

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 oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with the Tech 1. Ideal fuel trim values are around 128; if the oxygen sensor signal is indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 128. If a rich condition is detected, the fuel trim values will be below 128, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, the PCM will set DTC P0171.

WILL SET WHEN:
^ No DTC P0134, P0117, P0101, P0118, P0122, P0123, P0501 or P0502 set.
^ Engine coolant temperature between 80°C (176°F) and 110°C (230°F).
^ Throttle angle below 91%.
^ Vehicle speed less than 70 mph.
^ Engine speed between 1000 and 4000 RPM (VIN L) or 0 and 4000 RPM (VIN 1).
^ Purge duty cycle less than 80%.
^ Long term fuel trim above 158.
^ Short term fuel trim at 180.
^ Above conditions for over 25 seconds.

ACTION TAKEN (PCM will default to):
When DTC P0171 has initially been set, a software flag indicating an emissions-related fault will be set for 3 trips, after which the flag will clear provided the lean condition is no longer present. However, if conditions for setting DTC P0171 are detected during a trip while the flag is still set, the PCM will illuminate the MIL (Service Engine Soon). The MIL will remain illuminated for all subsequent trips until the DTC has been cleared by service or the rich condition has not been detected and conditions for checking fuel trim have been met for 3 consecutive trips.

NOTE: A trip is not considered complete until the fuel trim diagnostic has been run by the PCM. To complete a trip cycle for the fuel trim diagnostic, test must be enabled and run according to the "DTC P0171 Will Set When" criteria above.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Visually/physically checking items which may cause a lean condition may determine the cause of DTC P0171 being set.
2. A mass air flow sensor which causes the PCM to read a lower than actual air flow can cause a lean condition and DTC P0171 to be set.
3. Checks for a fault in the EVAP purge system which may cause DTC P0171 to set

DIAGNOSTIC AIDS
If a misfire can be felt and the problem cannot be isolated using this diagnostic chart, use CHART C-4, "Misfire Diagnosis." Chart C-4 Ignition Misfire Check
If the problem still cannot be isolated, try monitoring L.T. fuel trim and fuel trim cell while operating the vehicle under various loads. This may isolate the condition which caused P0171 to be set.