System Too Lean (Bank 1)
Long-term fuel trim on bank 1 is at the upper limit — the ECU is adding more fuel than baseline to maintain stoichiometric mixture but cannot reach target. Indicates unmetered air entering the intake or insufficient fuel delivery.
Affected components
- Intake manifold
- Mass air flow sensor
- Fuel pump
- Fuel pressure regulator
- Fuel injector
- Pcv valve
Common causes
- High likelihood: Vacuum leak in intake or PCV system
- High likelihood: MAF sensor contaminated, under-reading
- Medium likelihood: Fuel pressure below specification
- Medium likelihood: Fuel injectors partially clogged
Possible symptoms
- Check engine light on
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
Related codes
Sources
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.