Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is diagnosing a vehicle with an intermittent 'Check Engine' light and a stored DTC P0171 (System Too Lean Bank 1). The scan tool's live data shows that the Short Term Fuel Trim (STFT) for Bank 1 is consistently positive (+20% to +25%) at idle and during light cruise. What does this fuel trim reading indicate?
- AThe engine is running lean, and the PCM is adding fuel.
- BThe oxygen sensor is faulty, providing inaccurate readings.
- CThe engine is running rich, and the PCM is removing fuel.
- DThe engine is operating normally, and no action is needed.
Show answer & explanationAnswer & explanation
Correct answer: A. The engine is running lean, and the PCM is adding fuel.
A consistently positive Short Term Fuel Trim (+20% to +25%) indicates that the Powertrain Control Module (PCM) is adding a significant amount of fuel to Bank 1 because its oxygen sensor is detecting a lean condition (too much air, not enough fuel). The PCM is trying to compensate for this perceived lean state to maintain the ideal air-fuel ratio.
Why the other options are wrong
- B. While a faulty O2 sensor could cause incorrect fuel trims, the P0171 DTC specifically points to a system running lean, which the positive fuel trim confirms.
- C. Negative fuel trim indicates the PCM is removing fuel because it detects a rich condition.
- D. A +20% to +25% fuel trim is significantly outside the normal range (typically ±10%) and requires diagnosis.
Positive Fuel Trim
A positive fuel trim value indicates that the PCM is increasing the fuel injector pulse width (adding fuel) to compensate for a lean air-fuel mixture detected by the oxygen sensors.
- PCM adds fuel.
- Indicates a lean condition (too much air or too little fuel).
- Common causes: vacuum leaks, low fuel pressure, dirty injectors.
Memory trick: Positive trim adds more gas, if it's lean, it needs it fast.