Cause
Oil contamination from over-oiled air filter
Oil from a re-oilable performance air filter carried downstream onto the mass airflow sensor's element. The filter is designed to be oiled, and the fault is quantity: a filter re-oiled generously, or refitted before the oil has wicked in, sheds a mist into the intake.
A coated hot-wire element loses heat more slowly, so the sensor under-reports airflow and the module leans the mixture — and it does so progressively as the coating builds. A lean condition that arrived gradually on a vehicle with an aftermarket filter is this, and it is one of the few faults with a clear preventable cause.
Cleaning the element sometimes recovers it, with the manufacturer's approved cleaner and no contact with the wire, but a heavily contaminated sensor is usually replaced. The important half is the filter: a new sensor behind an over-oiled filter is contaminated again within weeks. Check the ducting for oil film while it is apart, since that confirms the source beyond doubt.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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