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A split in the ducting between the airflow sensor and the throttle, admitting air the engine never measured. The position matters more than the size — a leak here is unmetered air, so the mixture goes lean and the module's corrections chase a quantity that was never counted.

The split is usually where the duct flexes, on the underside of a bend or at a concertina section, and it often opens only under engine movement or at certain loads. So the fault comes and goes and reads as a failing sensor, which is exactly what makes the airflow sensor the most over-replaced part on the engine.

Fuel trims tell you a leak exists; only a smoke test tells you where. Positive trims at idle falling away under load is the classic pattern, because the leak is a large fraction of a small airflow and a small fraction of a large one. Flex the ducting by hand with the engine running — an idle that changes as the duct is squeezed or twisted locates it without any equipment.