Diagnostic Aids

Check for 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 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. Refer to Fuel System Diagnosis .
• 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. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
• A mass air flow (MAF) sensor output that is incorrect may cause the PCM to command the fuel system to go lean. Check the performance of the MAF sensor. Inspect for air restrictions and air inlet leaks near the MAF sensor. If the HO2S voltage is fixed low, while the fuel trim is constantly high, check the MAF sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition.
• Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving the connectors and 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 of the contamination and correct the condition before replacing the HO2S.
• Check for fuel contamination of the crankcase. Fuel that has entered the crankcase can release vapors that enter the engine through the positive crankcase ventilation (PCV) system, thereby causing a rich running condition.
• 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 mass air flow (MAF) sensor for faulty electrical connections to the PCM.

An intermittent malfunction may be caused by a fault in the HO2S 1 electrical circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

If 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.