A closed loop air/fuel metering system is used to provide the best combination of driveability, fuel economy, and emission control. In this system, short term fuel control is accomplished by the ECM rapidly responding to the oxygen sensor readings. These changes finely tune engine fueling. The long term fuel trim control is determined by the trends of the short term fuel trim. The ECM uses the long term fuel trim to make course adjustments to the engine fueling. Under normal driving conditions, except power enrichment mode, the ECM attempts to maintain a 14.7 to 1 air/fuel ratio. Fuel trim values of 0% are ideal. If the value is above 0%, this indicates the engine is running lean. Therefore the ECM commands more fuel to the system in attempt to maintain a 14.7 to 1 air/fuel ratio. If the value is below 0%, this indicates the engine is running rich. Therefore the ECM commands less fuel to the system in attempt to maintain a 14.7 to 1 air/fuel ratio. The ECM stores fuel trim values for three different speed/load ranges: idle, medium speed/load, and high speed/load. DTC P0171 will set if the long term fuel trim value exceeds a calibrated upper level in any of the above described speed/load ranges.

DTC P0171 will set if the vehicle is in closed loop and the following fuel trim limits have been exceeded:

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 Scan tool to monitor HO2S BANK 1 SENSOR 1 and HO2S BANK 1 SENSOR 2. If HO2S bank 1 sensor 1 lambda value is greater than 1.2 and HO2S bank 1 sensor 2 is below 300 mV, the system is running lean and the oxygen sensors are probably not at fault.