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 trim is accomplished by the ECM rapidly responding to the oxygen sensor readings. These changes finely tune engine fueling. Additionally, long term fuel trim is determined by trends in short term fuel trim, and serves as a coarse tuning mechanism for engine fueling. Fuel trim values near 0% are ideal. As the value increases, this indicates that the engine is running lean. Negative or decreasing values indicate a rich state of operation. The ECM in the vehicle stores fuel trim values for three different speed/load ranges: one at idle and another at medium speed/load, another at high speed/load. This code sets when the fuel trim is excessively high in any of the three speed/load ranges.

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

P0171 is a (type B) DTC.

The following are malfunctions that could cause a lean condition:

Important: If the front oxygen reading is greater than 1.2, and the rear HO2S is below 300 mV all the time, the O2 sensors are probably not at fa