All trouble codes → Lean fuel mixture codes
Lean fuel mixture codes
A “lean” code means the computer added fuel to make up for a mixture that read too lean, and ran out of room to correct it. The usual culprits are unmetered air sneaking in after the mass-air sensor, a sensor reading air wrong, or the fuel system not delivering enough. Comparing fuel trims at idle versus 2,500 rpm is the fastest way to split a vacuum leak from a fuel-delivery problem.
Fuel mixture fault — fix soon to avoid catalyst damage and poor fuel economy.
Common causes
- Vacuum/intake leak after the MAF (hoses, PCV, intake gaskets)
- Dirty or failing MAF sensor
- Low fuel pressure/volume (pump, filter, regulator)
- Restricted or leaking injectors
- Exhaust leak ahead of the front O2 sensor
First checks
- Compare short + long term trims at idle vs 2,500 rpm: worse at idle → vacuum leak; worse under load → fuel delivery or MAF
- Smoke-test the intake
- Check MAF g/s at idle (roughly engine litres × 1 g/s on many engines) and at 2,500 rpm
- Check fuel pressure against spec
System: Fuel & air metering. This is general guidance for the generic OBD-II codes in this group; always confirm against the service information for your specific vehicle.
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