To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in closed loop, the PCM monitors the pre-catalyst HO2S signals and adjusts fuel delivery based upon these voltages. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0 percent; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values more than 0 percent. If a rich condition is detected, the fuel trim values will be less than 0 percent, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the PCM will set this DTC. The PCM maximum authority to control long term fuel trim allows a range between -20 percent and +20 percent. The PCM s maximum authority to control short term fuel trim allows a range between -20 percent and +20 percent. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells are as follows:

The vehicle may have to be operated in all of the above conditions (marked with X) before the PCM determines fuel trim status. The amount of fuel trim deviates from 0 percent in each cell and the amount that each cell is used directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.

Important: Some conditions can cause multiple cells to indicate rich

Below are some examples of why a rich condition could exist in the above cells.

Inspect for the following:

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: :  Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

The numbers below refer to the step numbers on the diagnostic table.

2. This step determines whether the fault is present.

3. Using the Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and/or Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate the vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This will isolate when the DTC failed. For an intermittent condition, refer to Symptoms.

4. If both rich DTCs are set, both banks are operating rich. Inspect items that would cause both banks to operate rich.

5. Excessive fuel in the oil will cause DTCs P0172 and P0175 to set at the same time. Remove the PCV valve from the intake manifold and plug with a suitable stopper. Also disconnect the fresh air pipe from the rocker cover and plug using a suitable stopper. If the long term and short term fuel trim values increase, fuel in the oil is indicated.