Vanne de résonance du collecteur d'admission grippée

The valve that changes the effective length or path of the intake tract, jammed so that the manifold is stuck in one of its two configurations. What makes it awkward to diagnose is that the engine still runs well in half its rev range — whichever half matches the position the valve is stuck in — so the complaint is a flat spot rather than a fault, and it is easy to attribute to fuelling.

Long runners favour low-speed torque, short ones favour high-speed breathing, and the valve exists to give an engine both. Stuck long, the engine feels strong from idle and runs out of breath at the top. Stuck short, it feels hollow low down and comes alive late. The module detects it by commanding a change and watching manifold pressure or the position feedback fail to move, which is why the same seizure appears both as a stuck-open and a stuck-closed code depending on where it stopped.

Carbon is almost always the mechanism. Oil mist from the crankcase circuit and recirculated exhaust bake onto the shaft and its bore until the actuator cannot overcome the friction. So check the actuator's effort before condemning it — a healthy actuator against a seized shaft and a failed actuator against a free shaft produce the same code and different repairs.