DTC P0171: Fuel Trim System Lean: Description

The Heated Oxygen Sensor 1 (HO2S1) sends voltage signals that relate difference in oxygen between atmosphere and exhaust gas. Under normal driving conditions, except power enrichment mode, PCM attempts to maintain a 14.7:1 air/fuel ratio. PCM can make corrections from a nominal 128 short/long term fuel trim value (zero percent correction) within a maximum window of 70 to 165 (-45 to 29 percent correction) without setting DTC P0171 or DTC P0172. If PCM determines a rich condition exists (HO2S1 voltage more than 600 millivolts), it will decrease injector pulse width to maintain a 14.7:1 air/fuel ratio. Short term fuel trim value will be between 70 and 128. If PCM determines a lean condition exists (HO2S1 voltage less than 300 millivolts), it will increase injector pulse width to maintain 14.7:1 air/fuel ratio. Short term fuel trim value will be between 128 and 165. When certain conditions have been met (ECT and closed loop), PCM will go into learned 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 short term fuel trim value. If short term fuel trim value goes a certain number of counts in one direction, long term fuel trim value will update in the same direction, in the cell that corresponds with drive condition vehicle was in. After approximately 2 minutes of being in each of the long term fuel cells, PCM will enable EVAP purge solenoid (pulling fuel from 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 purge cells are used and sets when combined average or long term fuel cells for idle and cruise is more than calibrated long term fuel trim value.