Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairMedium
A technician is performing a diagnostic on a vehicle with a Check Engine Light (CEL) and a P0171 DTC (System Too Lean, Bank 1). The scan tool shows that the upstream oxygen (O2) sensor voltage is consistently low (around 0.1V - 0.2V), and both short-term fuel trim (STFT) and long-term fuel trim (LTFT) for Bank 1 are very high positive (+25% to +30%). After checking for vacuum leaks and finding none, the technician suspects a fuel delivery issue. What is the next logical step in diagnosis?
- APerform a fuel pressure test.
- BInspect the catalytic converter.
- CReplace the upstream O2 sensor.
- DCheck the exhaust for leaks.
Show answer & explanationAnswer & explanation
Correct answer: A. Perform a fuel pressure test.
A consistently low O2 sensor voltage combined with high positive fuel trims strongly indicates a lean condition. Since vacuum leaks were ruled out, the next most likely cause is insufficient fuel delivery. A fuel pressure test will confirm if the fuel pump, fuel filter, or pressure regulator is operating correctly and providing adequate fuel pressure to the injectors.
Why the other options are wrong
- B. A clogged catalytic converter would usually cause back pressure issues and potentially P0420/P0430 codes, not directly a P0171 with these specific fuel trim and O2 sensor readings.
- C. While a faulty O2 sensor can cause issues, the high positive fuel trims indicate the ECU is actively trying to compensate for a lean condition, suggesting the O2 sensor is likely reporting correctly. Replacing it without further diagnosis is premature.
- D. Exhaust leaks before the O2 sensor introduce unmetered air and would cause the O2 sensor to read lean, but the technician already checked for 'vacuum leaks,' which should have included exhaust leaks if they were before the O2 sensor. This question implies a focus on fuel delivery next.
Lean Condition Diagnosis Flow
Diagnosing a lean condition (P0171/P0174) involves systematically checking for vacuum leaks, exhaust leaks before the O2 sensor, and then fuel delivery issues (pressure, injectors, filter), followed by MAF sensor accuracy.
- P0171/P0174 indicates lean condition.
- Prioritize vacuum/exhaust leaks first.
- Next, verify fuel system integrity (pressure, delivery).
- Finally, check air metering (MAF/MAP sensor).
Memory trick: P0171 means 'Poor Fuel, Very Lean'. Check 'Air', then 'Fuel'.