A technician is diagnosing a vehicle with a complaint of poor fuel economy and a strong smell of raw fuel from the exhaust. The scan tool shows that the upstream oxygen (O2) sensor voltage is consistently high (around 0.8V - 0.9V) and both short-term fuel trim (STFT) and long-term fuel trim (LTFT) are very low negative (-20% to -25%). What is the most likely cause?
- AA faulty Mass Air Flow (MAF) sensor reporting low airflow.
- BA vacuum leak.
- CA leaking fuel pressure regulator diaphragm.
- DA restricted exhaust system.
Show answer & explanationAnswer & explanation
Correct answer: C. A leaking fuel pressure regulator diaphragm.
A consistently high O2 sensor voltage indicates a rich condition (excess fuel, not enough oxygen). Very low negative fuel trims mean the ECU is actively reducing fuel delivery to compensate for this rich condition. A leaking fuel pressure regulator diaphragm can allow fuel to be drawn directly into the intake manifold via the vacuum line, causing an excessive amount of fuel and thus a rich condition.
Why the other options are wrong
- A. A faulty MAF sensor reporting low airflow would cause the ECU to inject less fuel, leading to a lean condition (low O2V, high positive trims), opposite of the symptoms described.
- B. A vacuum leak introduces unmetered air, causing a lean condition (low O2V, high positive trims).
- D. A restricted exhaust system would primarily cause power loss and potentially P0420/P0430 codes, not a consistent rich condition with high O2V and negative trims.
Rich Condition Diagnosis Flow
Diagnosing a rich condition (P0172/P0175) involves systematically checking for excess fuel delivery (leaking injectors, high fuel pressure, faulty regulator) or inadequate air metering (faulty MAF/MAP sensor reporting too much air, restricted air intake).
- P0172/P0175 indicates rich condition.
- High O2 sensor voltage, negative fuel trims.
- Check for excess fuel (regulator, injectors, pressure).
- Check for incorrect air measurement (MAF/MAP).
Memory trick: High 'O2V' and 'Negative Trims' mean 'Too Much Fuel' or 'Too Little Air'.