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An injector that does not seal when closed, so fuel enters the cylinder outside the injection event. This produces a rich condition and hard starting rather than a misfire code on its own, and fuel washing down the bore dilutes the oil. This is the injector that has stopped sealing, as distinct from the common-rail injector whose seat is wearing — that one fails in both directions at once, over-delivering while open as well as passing fuel when closed, and it is a different measurement: a leak-back comparison across the set rather than a search for one dribbling injector.