What does it mean?
In closed loop the module continuously corrects injector delivery from oxygen sensor feedback, and that correction has a hard limit built into the calibration. This code is stored when the correction has been driven all the way to its maximum lean side and the mixture is still rich, meaning the module has run out of authority to fix it. Something is adding fuel or removing air outside the module's control, so the trims can no longer compensate. Because the mixture stays rich, catalyst temperature rises, fuel consumption climbs and the spark plugs suffer.
Symptoms
- Strong fuel smell and black smoke, particularly under acceleration
- Rough idle, hesitation and occasional misfire
- Sharply increased fuel consumption
- MIL on, with catalyst efficiency codes often appearing later
- Fouled spark plugs and difficult hot starting
- Failed emissions test with high carbon monoxide and hydrocarbon readings
Likely causes
- Leaking or dripping fuel injectors, or injectors whose flow characteristic has shifted
- Fuel pressure too high because of a stuck regulator, a restricted return line or a fuel pressure control fault
- Mass air flow or manifold pressure sensor overstating the incoming air
- Contaminated or shorted oxygen sensor reporting lean when the mixture is not
- Restricted air filter or induction path reducing the real airflow
- Engine oil diluted with fuel, or a crankcase ventilation system pulling fuel vapour into the intake
- Excessive purge flow from a saturated evaporative canister or a stuck open purge valve
How to fix it
- Record short and long term fuel trims at idle, cruise and load and confirm the correction really is pinned at its lean limit
- Check fuel pressure against specification with a gauge, both static and under load, and verify the return path is clear
- Compare the mass air flow or manifold pressure reading with the expected value for the engine speed and load to catch a sensor that overstates airflow
- Isolate the evaporative purge line and see whether the mixture corrects, which would point to a stuck purge valve or a saturated canister
- Pressure test the injectors for leakage and check the engine oil for fuel dilution
- Verify the oxygen sensor by forcing a lean and then a rich condition and watching how it responds
- Repair the cause, reset the fuel adaptation values and confirm the trims return toward normal on a road test