What does it mean?
Closed loop fuel control adds or removes fuel from the base calibration according to the oxygen sensor signal, but the correction range is limited. This code is stored when the module has commanded the maximum extra fuel the calibration allows and the exhaust still reads lean. That means air is entering the engine without being measured, or fuel delivery is falling short of what the module believes it is injecting. A mixture that stays lean raises combustion temperature and can damage the catalyst and other components if it is left unresolved.
Symptoms
- Hesitation, surging and a flat spot when pulling away or accelerating
- Rough or high idle, sometimes with an audible hiss from an intake leak
- Occasional misfire and a lack of power under load
- MIL on, often together with lean bank codes
- Higher exhaust temperature and, over time, catalyst damage
- Hard starting after the vehicle has stood
Likely causes
- Unmetered air entering after the air flow sensor through a split intake hose, a loose clamp, a leaking manifold gasket or a disconnected crankcase ventilation hose
- Low fuel pressure caused by a weak pump, a restricted filter or a leaking regulator
- Restricted or partly blocked injectors delivering less than commanded
- Mass air flow sensor under-reporting because it is contaminated or damaged
- Exhaust leak upstream of the oxygen sensor letting outside air reach the sensing element
- Failing fuel pump delivery volume that only shows up under load
- Vacuum leak at the brake booster, purge valve or a cracked vacuum line
How to fix it
- Record short and long term fuel trims at idle, cruise and load and confirm the correction is at its rich limit
- Smoke test the intake system from the air flow sensor to the manifold to find unmetered air, and check every hose and gasket
- Measure fuel pressure and delivery volume against specification, both at idle and under load
- Compare the air flow sensor reading with the expected value for that engine speed and load
- Carry out an injector balance or flow test to identify restricted injectors
- Inspect the exhaust ahead of the oxygen sensor for leaks that can produce a false lean signal
- Repair the cause, reset the fuel adaptations and confirm on a road test that the trims settle back toward normal