System Too Lean (Bank 1)
Common Symptoms
- Check engine light on
- Rough idle or hesitation
- Poor fuel economy
- Engine may surge at idle
- Hard starting when cold
Most Likely Root Causes
- Vacuum leak (intake manifold gasket, PCV hose, brake booster)
- Dirty or failing mass airflow (MAF) sensor
- Weak or failing fuel pump
- Clogged fuel injectors
- Faulty fuel pressure regulator
- Exhaust leak near upstream O2 sensor
- Failing upstream O2 sensor
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Severity / Derate Risk
Moderate — lean conditions can cause engine damage over time. Address promptly.
Component Location
MAF sensor located in air intake duct between air filter and throttle body. O2 sensor Bank 1 upstream is in exhaust manifold or downpipe.
Manufacturer Notes
Always check fuel trims at idle AND at 2500 RPM. Idle-only lean often indicates vacuum leak. Cruise-only lean often indicates MAF or fuel delivery issue.
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