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) code. Live data shows that Short Term Fuel Trim (STFT) is consistently +25% and Long Term Fuel Trim (LTFT) is also significantly positive. The fuel pressure is within specifications. What is the MOST likely cause?
- AA vacuum leak on Bank 1 affecting unmetered air
- BA leaking fuel injector on Bank 1
- CA restricted exhaust on Bank 1
- DA faulty Bank 1 upstream oxygen sensor reading rich
Show answer & explanationAnswer & explanation
Correct answer: A. A vacuum leak on Bank 1 affecting unmetered air
Consistently high positive fuel trims (+25% STFT and high positive LTFT) indicate the engine control module (ECM) is adding a significant amount of fuel to compensate for a lean condition. With fuel pressure confirmed good, a vacuum leak introducing unmetered air into Bank 1 is a very common cause of such symptoms.
Why the other options are wrong
- B. A leaking fuel injector would cause a rich condition (negative fuel trims), not a lean condition.
- C. A restricted exhaust would typically cause a lack of power and potentially rich conditions due to inefficient scavenging, not a lean condition with positive fuel trims.
- D. If the upstream O2 sensor was faulty and reading rich, the ECM would reduce fuel (negative fuel trims), not add it.
Positive Fuel Trim
The engine control module (ECM) adding fuel to the mixture, indicated by a positive percentage in Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values.
- ECM adding fuel to compensate
- Caused by a lean condition
- Common causes: vacuum leaks, low fuel pressure, faulty MAF
Memory trick: Air's too much, fuel's too little, trims add a lot.