Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a vehicle with an intermittent 'Check Engine' light and a stored P0171 (System Too Lean Bank 1) diagnostic trouble code. Live data from the scan tool shows Short Term Fuel Trim (STFT) fluctuating rapidly between +10% and +20% and Long Term Fuel Trim (LTFT) at +18%. The technician suspects a vacuum leak. What would be the MOST appropriate next diagnostic step?
- ACheck fuel pressure and volume.
- BReplace the upstream oxygen sensor.
- CInspect for exhaust leaks before the upstream oxygen sensor.
- DPerform a smoke test on the intake system.
Show answer & explanationAnswer & explanation
Correct answer: D. Perform a smoke test on the intake system.
High positive fuel trims (both STFT and LTFT) indicate the engine control module (ECM) is adding a significant amount of fuel to compensate for a lean condition. A vacuum leak is a common cause of a lean condition, especially at idle or low RPMs, and a smoke test is an effective method to locate such leaks.
Why the other options are wrong
- A. Low fuel pressure/volume would cause a lean condition, but a smoke test is more specific to vacuum leaks given the context.
- B. Replacing the O2 sensor without confirming its fault is premature; the sensor is likely reporting the lean condition accurately.
- C. Exhaust leaks would cause the oxygen sensor to read lean, but a vacuum leak is a more direct cause for the positive fuel trims.
Positive Fuel Trim Diagnosis
A diagnostic approach used when the engine control module (ECM) is adding fuel (positive fuel trim) to compensate for a lean condition, indicating too much air or too little fuel.
- High positive STFT and LTFT point to a lean condition.
- Common causes include vacuum leaks, low fuel pressure, clogged injectors, or unmetered air.
- Diagnosis involves systematically checking for these causes.
Memory trick: Positive trims mean the engine's thirsty; find the 'air' hole with smoke.