P0171

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 101 to 148 (-21% to 15% 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 101 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 148. When certain conditions have been met (ECT and closed loop), the PCM will go into a leaned fuel control mode by updating and using four long term fuel trim cells: 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 PARAMETERS
DTC P0171 will set if the combined average of the idle and cruise long term fuel trim cells is greater than 148 (greater than 15.4% correction) and the current short term fuel trim cell is greater than 128 when:
^ Condition exists for longer than 3 seconds
^ PCM is using non-purge fuel trim cells
^ Engine speed is between 500 and 4000 RPM
^ Loop status is closed
^ ECT is between 60° than 115°C (1400 and 259°F)
^ IAT is between 20° and 80°C (100 and 176°F)
^ MAP is between 30 kPa and 90 kPa
^ TP angle is less than 75%
^ Barometric pressure is greater than 70 kPa
^ Calculated air flow is between 2 and 80 gm/s
^ No cam, CKP, ECT, EGR flow, EVAP purge solenoid, IAC valve, IAT MAP misfire, O2S-1 or TP sensor DTCs have been set

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

DTC P0171 is a type A DTC.

DIAGNOSTIC AIDS
DTC P0171 indicates the engine is running lean. The following are possible causes of a lean air/fuel mixture:
^ Vacuum leak
^ Fuel pressure low
^ Fuel quality poor or water in fuel
^ Exhaust manifold gasket leak or leak around
^ O2S-1
^ Fuel injectors clogged
^ O2S-1 weak

To verify the system is running lean, use 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.