P0171

DTC P0171 System Too Lean (Bank 1, Sensor 1) (Part 1 Of 2):

DTC P0171 System Too Lean (Bank 1, Sensor 1) (Part 2 Of 2):

Heated Oxygen Sensor (HO2S 1) (Bank 1, Sensor 1):

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 Vehicle Control Module-A (VCM-A) 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 VCM-A 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 VCM-A is reducing the amount of fuel delivered. If an excessively lean condition is detected, the VCM-A will set DTC P0171. DTC P0171 is a type "B" DTC.

DTC P0171 WILL SET WHEN
^ No Manifold Absolute Pressure (MAP) DTC's set.
^ No Heated Oxygen Sensor (HO2S) DTC's set.
^ No Exhaust Gas Recirculation (EGR) DTC's set.
^ No Engine Coolant Temperature (ECT) DTC's set.
^ No Evaporative Emissions (EVAP) System DTC's set.
^ No Intake Air Temperature (IAT) DTC's set.
^ No Vehicle Speed Sensor (VSS) DTC's set.
^ Throttle position is less than 95%.
^ Engine speed is between 575 RPM and 4500 RPM.
^ Barometric Absolute Pressure (BARO) is greater than 70 kPa (21 Hg).
^ Engine Coolant Temperature (ECT) is between 0°C and 100°C (32°F and 212°F).
^ Manifold Absolute Pressure (MAP) is between 20 kPa and 99 kPa (6 Hg and 29 Hg).
^ Intake Air Temperature (IAT) is between -30°C (-22°F) and 70°C (158°F).
^ Vehicle speed is less than 85 mph.
^ The average of short term fuel trim samples is no more than 115.

ACTION TAKEN (VCM-A 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 VCM-A will turn "ON" the Malfunction Indicator Lamp (MIL) (Service Engine Soon). The MIL will remain "ON" for all subsequent trips until the DTC has been cleared by service or the lean condition has not been detected and conditions for checking fuel trim have been met for 3 consecutive trips.

DTC P0171 WILL CLEAR WHEN
The VCM-A will turn the MIL "OFF" after three consecutive trips without a fault condition present. A history DTC will be cleared if no fault conditions have been detected for forty warm-up cycles [coolant temperature has risen 22°C (40°F) from start-up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle] or the Tech 1 clearing feature has been used.

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

DTC CHART 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 the DTC being set.
2. Checking fuel delivery system for proper operation.
3. Checks for a fault in the EVAP purge system which may cause DTC P0171 to be set.

DIAGNOSTIC AIDS
If the problem cannot be isolated using the diagnostic charts, try monitoring Long Term (L.T.) fuel trim and fuel trim cell while operating the vehicle under various loads. This may isolate the condition which caused DTC P0171 to be set.

A condition that causes DTC P0300 (Misfire Detected) may also set DTC P0171; conversely, an extremely lean condition which sets DTC P0171 can cause a misfire at idle and DTC P0300 to set. If the cause of DTC P0171 cannot be determined and DTC P0300 is also set, refer to DTC P0300 Random Misfire Detected. P0300