System Too Lean (Bank 1)

Possible causes

• CommonVacuum leak after the mass airflow sensor - split hose, failed gasket, leaking PCV
• CommonDirty or failing mass airflow sensor under-reporting airflow
• PossibleWeak fuel pump or clogged fuel filter
• PossibleClogged or partially blocked fuel injectors
• PossibleExhaust leak before the oxygen sensor, letting air in and fooling it
• PossibleFailing oxygen sensor reporting lean when the mixture is correct
• PossibleLow fuel pressure from a failing pressure regulator

What you might notice

• Rough or hunting idle
• Hesitation or a flat spot under acceleration
• Check engine light on
• Sometimes no noticeable symptom at all
• Slightly reduced fuel economy despite the lean mixture

What to check, in order

1. Look for a vacuum leak first - this is the most common cause by a wide margin. With the engine idling, listen for a hiss around intake hoses, the PCV system and the intake manifold gasket.
2. Watch the fuel trim values on the live data screen. Long-term trim strongly positive at idle but returning to normal at higher engine speed points at a vacuum leak, because the leak matters less when the throttle is open.
3. Inspect the mass airflow sensor element. A dirty sensor under-reads airflow, so the module injects too little fuel.
4. Check for exhaust leaks between the engine and the oxygen sensor. Air drawn in through a leak makes a correct mixture look lean.
5. If trim is positive across all engine speeds and loads, suspect fuel delivery - test fuel pressure against the manufacturer's specification.
6. A persistent lean mixture can raise combustion temperatures. Do not ignore this code for long, particularly on a turbocharged engine.