P0172

A closed loop air/fuel metering system is used to provide the best combination of drive ability, 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 this vehicle store fuel trim values for three different speed/load ranges: one at idle, another at medium speed/load, and another at high speed/load. This code sets when the fuel trim is excessively low in any of the three speed/load range.

DTC PARAMETERS
This DTC will set if closed loop is active, and the fuel trim limits values are below the limits listed below:
^ Idle: less than -7.7%
^ Medium speed/load: less than -24%
^ High speed/load: less than -21%

DTC P0172 is a (type B) DTC

DIAGNOSTIC AIDS
The following are malfunctions that could cause a rich condition:
^ Fuel pressure high
^ Leaking fuel injector(s)
^ Vacuum leak around MAP sensor
^ Oxygen sensor contamination
^ Restricted exhaust or air intake
^ EGR valve stuck open
^ EVAP purge solenoid stuck open

IMPORTANT: If the front oxygen reading is less than 0.8, and the rear HO2S is below 300 mV all the time, the O2 sensors are probably not at fault.