Débitmètre d'air défectueux

The sensor that measures how much air is entering the engine, and the single most over-replaced part in the intake. It works by holding a heated element at a fixed temperature above the incoming air and reporting the current needed to do it — more air carries away more heat, so current is a proxy for mass flow.

The reason it fails is usually contamination rather than breakage. Oil misted from the crankcase breather, dust past a poorly seated filter, or residue left by an over-oiled performance filter coats the element. A coated element loses heat more slowly, so the sensor under-reports airflow — and it does so progressively, which matters diagnostically: a sensor that has drifted cannot be the cause of a fault that appeared abruptly.

It is also the part most often blamed for somebody else's fault. Unmetered air entering after the sensor — a split duct, a loose clamp, a perished hose — makes the engine take in more air than was measured, and the code that results names the measurement rather than the leak. A smoke test finds that in minutes and costs less than the sensor. Check fuel trims and the ducting before condemning it.