Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A technician is diagnosing a vehicle with an intermittent P0174 (System Too Lean Bank 2) DTC. Live data shows that the long-term fuel trims (LTFT) for Bank 2 are consistently at +20% at idle and cruising speeds, while Bank 1 LTFTs are around +5%. Which of the following is the MOST likely cause?
- AA faulty Bank 1 oxygen sensor.
- BA restricted exhaust manifold on Bank 2.
- CA clogged fuel injector on Bank 2.
- DAn exhaust leak on Bank 2 before the oxygen sensor.
Show answer & explanationAnswer & explanation
Correct answer: C. A clogged fuel injector on Bank 2.
A +20% LTFT indicates the engine control module (ECM) is adding a significant amount of fuel to Bank 2 because it detects a lean condition. If this is specific to one bank and consistent across different engine loads, a clogged fuel injector on that bank is a strong possibility, as it would directly reduce fuel delivery to those cylinders, making the ECM try to compensate.
Why the other options are wrong
- A. A faulty Bank 1 oxygen sensor would affect Bank 1 fuel trims, which are reported as normal (+5%).
- B. A restricted exhaust manifold would likely cause a rich condition or misfires, not a lean condition that the ECM tries to compensate for with positive fuel trims.
- D. An exhaust leak before the oxygen sensor would introduce ambient air, causing the O2 sensor to read lean, leading to positive fuel trims. However, clogged injectors are a more direct cause of lean conditions that persist across loads and are specific to one bank.
Positive Fuel Trims (Lean)
Positive fuel trims indicate the Engine Control Module (ECM) is adding more fuel than calculated because the oxygen sensors detect a lean condition.
- Corrects for insufficient fuel delivery or excess air.
- Can be short-term (STFT) or long-term (LTFT).
- High positive trims (>+10% to +20%) often indicate a fault.
Memory trick: Bank's lean, fuel trims keen, injector scene.