In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a closed loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in closed loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.

Check for any of the following conditions:

If the DTC P0172 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0172 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0172 appears in the Not Ran Since Code Cleared list, the P0172 diagnostic has not yet run. When the DTC P0172 does not appear in the Not Ran Since Code Cleared list, the P0172 diagnostic has run. If the MIL is NOT ILLUMINATED and there is no PENDING DTC Status in DTC Information, the P0172 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

If you cannot duplicate a DTC P0172, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

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

1. The Powertrain (OBD) System Check prompts the technician to complete some basic checks and to store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step checks whether the fault is still present.

3. This step checks for a Total Fuel Trim value that is a negative number that is numerically higher than -22 percent, such as -23 percent, or -24 percent. Total Fuel Trim is a combined value of Short Term FT and Long Term FT.

8. If the condition is still present, any one of these sensors could cause this DTC to set. Check the performance of the MAF sensor, the ECT sensor, the TP sensor and the CTP switch signal. Repair any faulty conditions.