تفحم مقعد الصمام

Carbon on a valve's seating face, so it no longer closes against it. The seat is where sealing actually happens, and a deposit there means the valve reports closed while continuing to pass — position feedback measures the actuator, not the seal.

A valve that leaks continuously affects the engine most where flow is smallest, so the symptom is worst at idle and proportionally invisible under load. Check the seat by hand with the valve removed rather than trusting reported position. Cleaning is a repair with a lifespan: the deposit returns unless what produces it changes, so a vehicle doing short cold journeys will meet this again.