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The line returning unused reductant to the tank, blocked so the fluid cannot circulate. Not every system has one, so establish the plumbing first — on those that do, the return path also carries fluid away from the injector when dosing stops, which is what stops it crystallising in place.

A blocked return therefore causes the deposits that then block everything else. Fluid left standing in a hot injector dries and crystallises, and the injector that should have been purged is coated. So a return-line fault presents later as an injector or dosing fault, and replacing the injector without clearing the return means the new one crystallises too.

Crystallised reductant is usually what blocks it, which makes the fault self-reinforcing, along with freezing in cold weather since the fluid freezes at around minus eleven degrees. Check whether the line is genuinely blocked or merely frozen before dismantling — a system that works normally once thawed had ice, not a blockage, and the real question then is why the tank and line heaters did not run.