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On air-assisted systems, the pump supplying compressed air to atomise the reductant, faulty so the air is missing or at the wrong pressure. It is distinct from the reductant pump, which moves fluid — a system with air assistance has both, and confusing them sends the work to the wrong component.

Atomisation is what the air is for, and losing it does active harm. Fluid delivered as a stream rather than a mist lands on surfaces below its evaporation temperature and crystallises into a hard deposit. So an air pump fault produces deposits in the injector and on the catalyst face that outlast the original fault and become a second repair.

The module usually stops dosing rather than dosing badly, which is the correct choice and means the immediate symptom is a conversion efficiency fault rather than deposits. Check the air line and its filter before the pump — a restriction produces low pressure with a healthy pump, and these pumps draw through a small filter that blocks. Crystallised reductant in the air line itself is a common finding and means fluid has travelled the wrong way up it.