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Carbon build-up jamming a valve in the exhaust or emissions system, so it no longer moves through its commanded travel. It is the commonest mechanical failure in this part of the engine and it applies across several valves — recirculation cooling, secondary air, exhaust flaps — which is why the entry names the mechanism rather than one part.

Where it seizes decides the symptom, and it is rarely fully stuck. A valve stiff rather than jammed moves slowly, overshoots, or reaches only part of its travel, so the module reports a performance or range fault instead of a clean failure. A valve that works when exercised by hand and fails in service is stiff, not free — the actuator can overcome the friction on a bench and not against a spring under vibration.

Cleaning is a repair with a lifespan, and that is worth saying plainly. The deposit will return unless what produces it changes, so a vehicle doing short cold journeys will meet this again. Check the actuator's current draw while it operates if the tooling allows: a motor working unusually hard against a valve that reaches its position is the early warning, before any code appears.