System Too Lean at Idle (Bank 1)
Fuel trim on bank 1 hits the lean limit only at idle, while trim returns to normal at higher load. This pattern strongly indicates a vacuum leak — leaks become a larger fraction of total airflow at idle and so dominate the trim, but disappear into background noise above idle.
Affected components
- Intake manifold
- Pcv valve
- Brake booster
- Vacuum hose
Common causes
- High likelihood: Vacuum leak (intake gasket, brake booster, hoses)
- Medium likelihood: PCV valve stuck open
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.