P0174

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 in a lean state. Smaller 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 PARAMETERS
This DTC will set if closed loop is active, and the following fuel trim limits lave been exceded:
^ Idle: 7.7%
^ Medium speed/load: 21%
^ High speed/load: 21%

DTC P0174 is a (type B) DTC

DIAGNOSTIC AIDS
The following are malfunctions that could cause a lean condition:
^ Vacuum leak
^ Fuel pressure low
^ Poor fuel quality/water in fuel
^ Exhaust leakage around manifold or front 02S sensor
^ Fuel injectors clogged

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 fault.