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 Diagnostic Trouble Code (DTC) for 'System Too Lean (Bank 1)'. Using a scan tool, the technician observes that the Short Term Fuel Trim (STFT) for Bank 1 is consistently around +15% and the Long Term Fuel Trim (LTFT) for Bank 1 is around +10%. What do these fuel trim values MOST likely indicate?
- AThe oxygen sensor is faulty, providing inaccurate readings.
- BThe engine is running rich, and the PCM is removing fuel.
- CThe engine is running lean, and the PCM is adding fuel.
- DThe catalytic converter is inefficient, requiring replacement.
Show answer & explanationAnswer & explanation
Correct answer: C. The engine is running lean, and the PCM is adding fuel.
Positive fuel trim values (+15% STFT, +10% LTFT) indicate that the Powertrain Control Module (PCM) is adding fuel to the engine in an attempt to compensate for a lean condition. The PCM detects too much oxygen in the exhaust (lean condition) and therefore increases fuel delivery to bring the air-fuel ratio back to stoichiometric (14.7:1).
Why the other options are wrong
- A. While a faulty oxygen sensor can cause incorrect fuel trims, the consistent positive values suggest the O2 sensor is accurately reporting a lean condition, and the PCM is responding accordingly.
- B. Negative fuel trim values indicate the PCM is removing fuel because the engine is running rich.
- D. Catalytic converter inefficiency is typically indicated by specific DTCs and O2 sensor readings (e.g., downstream O2 mimicking upstream), not directly by positive fuel trims.
Positive Fuel Trims
Positive fuel trim values (STFT, LTFT) indicate that the Powertrain Control Module (PCM) is adding fuel to the engine to correct a lean air-fuel mixture, as detected by the oxygen sensors.
- PCM adds fuel to compensate for excess oxygen in exhaust.
- Common causes include vacuum leaks, low fuel pressure, dirty injectors, unmetered air.
- Values above +/- 10% often indicate a problem.
Memory trick: Positive trim means fuel's a must, to fix the lean, we trust.