P0171

The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM to control a 14.7 to 1 air/fuel ratio under normal driving conditions. The PCM can make fuel corrections from a nominal 128 (0% correction) short/long term fuel trim value to a maximum window of 101 to 147 (-21% to 15%). If the PCM determines a rich condition exists (02 voltage above 450 mV), it will decrease injector pulse width (reducing fuel) to maintain a 14.7 to 1 air/fuel ratio. The short term fuel trim value will be between 101 and 128 indicating a rich condition. If the PCM determines a lean condition exists (02 voltage below 450 mV), it will increase injector pulse width (adding fuel) maintaining a correct air/fuel mixture. The short term fuel trim value will be between 128 and 147 indicating a lean condition. When certain conditions have been met to begin learning fuel control (ECT closed loop, etc.) the PCM will use the short term fuel trim cell to update one of the four long term cells (idle, decel, cruise, accel) being used. During every drive cycle (trip) the PCM will use and update the idle and cruise long term cells and will keep track of how long the control system is operating in these cells. After approximately two minutes of being in each of these cells the fuel control system will enable EVAP purge (EVAP purge On) and the purge long term cells will be used. (Refer to EVAP Canister Purge section of this manual for information on purge long term fuel cells.) If the average of the idle and cruise fuel cells is 147 and the short term fuel trim cell is currently over 128, a lean fuel system DTC will set. This test is performed before the purge cells are used.

DTC PARAMETERS
DTC P0171 will set if the averaged long term idle and cruise fuel trim cells are 147 and the current short term fuel trim cell is above 128 for three seconds when:
^ PCM is using the non-purge long term fuel trim cells.
^ The system is in closed loop.
^ Engine coolant is between 60° and 115°C (140° and 239°F).
^ Air flow is between 2 and 80 grams/second.
^ Manifold pressure is between 20-90 kPa (3-13 psi).
^ RPM is between 500-4000 rpm.
^ TP is less than 75% open.
^ Barometer is greater than 70 kPa (10 psi).
^ IAT reading is between 300 and 80°C (-22°F and 176°F).
^ Vehicle speed is less than 121 km/h (75 mph).
^ No ECT, MAP, IAT, TP Sensor,VSS, EVAP Canister Purge, Front O2S or P0125 DTCs have been set.

IMPORTANT: The fuel control system is continuously checked when the above conditions have been met.

Possible Causes
The following are malfunctions that could cause a lean condition:
^ Vacuum leak
^ Fuel pressure too low. (Refer to "Engine/Fuel/Air Intake/Exhaust Service Manual" for procedure)
^ Water in the fuel
^ Exhaust leakage around manifold or front O2S sensor
^ Injector plugging.

IMPORTANT: If the front oxygen reading voltage is below 300 mV and the rear HO2S-2 is below 300 mV all the time, the O2 sensors are probably not at fault.