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A particle or corrosion between a valve and its seat, preventing it closing. It is distinct from carbon fouling — that builds gradually and this arrives at a moment — and distinct from a seized spindle, because the valve still moves freely.

A valve that cannot seal passes continuously, so the symptom is worst where flow is smallest, which is idle. The module may report the valve as closed, since position feedback measures the actuator rather than the seal. Check the seat by hand with the valve out rather than trusting the reported position. Where the debris came from matters: a particle in an evaporative or recirculation valve usually means something upstream is shedding, so cleaning the valve alone leaves the source in place.