Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is using a scan tool to diagnose a vehicle with an intermittent 'Check Engine' light. The scan tool shows a pending code P0171 (System Too Lean Bank 1). After reviewing live data, the technician observes that the short-term fuel trim (STFT) for Bank 1 intermittently spikes to +25% at idle, while the long-term fuel trim (LTFT) is consistently around +15%. What is the most likely cause of these fuel trim readings?
- AA leaking fuel injector on Bank 1.
- BA vacuum leak on Bank 1.
- CA faulty oxygen sensor on Bank 1, indicating rich.
- DAn exhaust restriction on Bank 1.
Show answer & explanationAnswer & explanation
Correct answer: B. A vacuum leak on Bank 1.
High positive fuel trims (STFT spiking to +25%, LTFT at +15%) indicate the engine control unit (ECU) is adding fuel because it detects a lean condition. A vacuum leak is a common cause for this, especially when it's more pronounced at idle (higher vacuum) and can be intermittent due to engine movement or temperature changes affecting rubber hoses.
Why the other options are wrong
- A. A leaking fuel injector would cause a rich condition, leading to negative fuel trims.
- C. A faulty O2 sensor indicating rich would cause the ECU to remove fuel, leading to negative fuel trims. If it were stuck lean, it would also cause positive trims, but the 'indicating rich' part makes this incorrect.
- D. An exhaust restriction would primarily affect engine performance and could cause a rich condition, not a lean one requiring more fuel.
Positive Fuel Trim
The engine control unit (ECU) adding fuel to compensate for a lean condition, indicated by positive percentages in short-term (STFT) or long-term (LTFT) fuel trims.
- Positive trims mean the ECU is adding fuel.
- Indicates a lean condition (too much air, not enough fuel).
- Common causes include vacuum leaks, low fuel pressure, dirty MAF sensor, exhaust leaks before O2 sensor.
Memory trick: Positive trims mean 'Plus Fuel' needed.