Diagnostic Aids

Check for any of the following conditions:

• Vacuum leaks.
• Exhaust leaks.
• Correct fuel pressure. Fuel system will be lean if fuel pressure is too low. To determine cause of DTC P0171, monitoring fuel pressure while driving vehicle at various road speeds may be necessary. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
• Check for fuel contamination. Small amounts of water can be delivered to fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. See ALCOHOL/CONTAMINANTS-IN-FUEL DIAGNOSIS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
• A Mass Air Flow (MAF) sensor output that is incorrect may cause PCM to command fuel system to go lean. Check performance of MAF sensor. Check for air restrictions and air inlet leaks near MAF sensor. If HO2S voltage is fixed low, while fuel trim is constantly high, check MAF sensor signal circuit for resistance. High circuit resistance can cause sensor to read a lower volume of air than actual, resulting in a lean fueling condition.
• Check wiring harness for damage. If harness appears to be OK, observe HO2S 1 display on scan tool while moving connectors and wiring harnesses related to sensor. A change in display will indicate location of fault.
• Check for a contaminated oxygen sensor. Silicone and other contaminants can cause oxygen sensor to provide false lean exhaust readings. If contamination is found, determine cause of contamination and correct the condition before replacing HO2S.
• Check for fuel contamination of the crankcase. Fuel that has entered the crankcase can release vapors that enter the engine through the PCV system, thereby causing a rich running condition.
• Check HO2S 1 for water intrusion into wiring harness and sensor housing. Water can create a short to ground in HO2S signal circuit, causing a false lean indication.
• Check HO2S 1 and MAF sensor for faulty electrical connections to PCM.

An intermittent malfunction may be caused by a fault in HO2S 1 electrical circuit. Check wiring harness and components for an intermittent condition. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 1.8L VIBE article. Repair any electrical circuit faults found. If DTC P0171 cannot be duplicated, information included in freeze frame data can be useful in determining vehicle operating conditions when DTC was first set.