Diagnostic Aids

Check any of the following conditions:

• Check for vacuum leaks.
• Check for exhaust leaks.
• Check for the correct fuel pressure. The fuel system will be lean if the fuel pressure is too low. In order to determine the cause of DTC P0171 monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary.
• Check for fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel.
• Malfunctioning MAF sensor.
• Check for a damaged wiring harness. Inspect the wiring harness for damage. If the harness appears to be okay, observe the HO2S 1 display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.
• Check for a contaminated oxygen sensor. Silicone and other contaminants can cause the oxygen sensor to provide false lean exhaust readings. If contamination is found, determine the cause and correct the condition before replacing the HO2S.
• Inspect the HO2S 1 for water intrusion into the wiring harness and the sensor housing. Water can create a short to ground in the HO2S signal circuit causing a false lean indication.
• Check the HO2S 1 and the MAF sensor for a faulty electrical connection to the PCM.

If a DTC P0171 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 P0171 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0171 appears in the Not Ran Since Code Cleared list, the P0171 diagnostic has not yet run. When the DTC P0171 does not appear in the Not Ran Since Code Cleared list, the P0171 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the P0171 diagnostic has passed. The 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 a DTC P0171 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.