DTC P0171: Fuel Trim System Lean: Description

Oxygen Sensor 1 (O2S1) 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 zero percent short/long term fuel trim value within a maximum window of -21 percent to 15 percent correction without setting DTC P0171 or P0172. If PCM determines a rich condition exists (O2S1 voltage greater than 600 millivolts), it will decrease injection pulse width to maintain a 14.7:1 air/fuel ratio. Short term fuel trim value will be -21-0 percent. If PCM determines a lean condition exists (O2S1 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 0-15 percent. When certain conditions have been met (ECT and closed loop), PCM will go into a learned fuel control mode by updating and using 4 long term fuel trim cells (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 same direction, in cell that corresponds with drive condition vehicle was in. After approximately 2 minutes of being in each 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. DTC is run before purge cells are used and sets when combined average of long term fuel trim cells for idle and cruise is greater than calibrated long term fuel trim value.