The oxygen sensor 1 (O2S-1) sends voltage signals that relate the difference in oxygen between the atmosphere and exhaust gas. Under normal driving conditions, except power enrichment mode, the PCM attempts to maintain a 14.7 to 1 air/fuel ratio. The PCM can make corrections from a nominal 128 short/long term fuel trim value (0% correction) within a maximum window of 70 to 165 (-45% to 29% correction) without setting DTC P0171 or P0172. If the PCM determines a rich condition exists (O2S-1 voltage above 600 mV), it will decrease injector pulse width to maintain a 14.7 to 1 air/fuel ratio. The short term fuel trim value will be between 70 and 128. If the PCM determines a lean condition exists (O2S-1 voltage below 300 mV), it will increase injector pulse width to maintain a 14.7 to 1 air/fuel ratio. The short term fuel trim value will be between 128 and 165. When certain conditions have been met (ECT and closed loop), the PCM will go into a leaned fuel control mode by updating and using a total of 22 long term fuel trim cells for idle, cruise, accel and decel. These cells update based on the short term fuel trim value. If the short term fuel trim value goes a certain number of counts in one direction, the long term fuel trim value will update in the same direction, in the cell that corresponds with the drive condition the vehicle was in. After approximately two minutes of being in each of the long term fuel trim cells, the PCM will enable the EVAP purge solenoid (pulling fuel from the EVAP canister) and a new set of long term fuel trim cells called purge long term fuel trim cells will be used. However, DTC P0171 is run before the purge cells are used and sets when the combined average of the long term fuel trim cells for idle and cruise is above the calibrated long term fuel trim value.

DTC P0171 will set if the LTFT value is greater than 165 (greater than 29% correction) when:

DTC P0171 diagnostic runs continuously once the above conditions have been met.

P0171 is a (type B) DTC

DTC P0171 indicates the engine is running lean. The following are possible causes of a lean air/fuel mixture:

To verify the system is running lean, use the scan tool to monitor O2S-1 and HO2S-2. If both sensors are below 300 mV, the system is running lean and the oxygen sensors are probably not at fault.