AdBlue-Filter verstopft

The filter in the reductant supply, restricted by crystallised deposits or contamination. It exists because the injector meters fluid through very fine passages — anything carried into them blocks the injector instead, which is a far more expensive part.

What blocks it is usually the fluid itself. Reductant crystallises as it dries, and fluid drawn from an open or part-used container carries dust and crystals in with it. Topping up from a jerry can kept in a workshop is a common route.

Its symptoms depend on how far gone it is, which is worth knowing because the codes differ: a partial restriction produces low pressure or high consumption, while a complete block produces no flow at all. So a pressure code and a no-flow code can be the same filter at two stages.

It is a service item on many vehicles and routinely missed — worth asking about specifically on a diesel with dosing faults and no service history for it.

Replacing it without addressing the fluid repeats the failure. Buy reductant in sealed containers, check the date, and do not decant it — the fluid has a genuine shelf life and degrades in heat and sunlight.