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Hard urea deposits built up on the injector's needle and seat until it no longer moves correctly. The mechanism explains why it is so common and what the real repair is. The injector sprays reductant into a hot exhaust; when the engine stops, the tip stays hot and the fluid left in it dries rather than evaporates, leaving solid deposits exactly where the needle has to move. Every heat cycle adds a little more.

It fails in both directions, which is why one cause serves two opposite codes. Deposits can jam the needle shut, so nothing is dosed and the emissions system escalates; or hold it off its seat, so fluid dribbles continuously, crystallises inside the exhaust and blocks the catalyst face.

Manufacturers fight it two ways, and a failure in either is usually the real fault. The injector is cooled, normally by engine coolant through its body, and the system purges fluid back to the tank after shutdown so nothing is left in the tip. A blocked coolant passage or a failed purge crystallises a replacement injector as quickly as it did the original — so ask why it happened before fitting a new one.

Cleaning is worth asking about. Urea deposits dissolve in warm water, and on some vehicles a crystallised injector can genuinely be recovered.