Línea de aire de inyección del AdBlue obstruida

On air-assisted reductant systems, a restriction in the compressed air line that atomises the fluid before it enters the exhaust. Only some systems use air assistance, so establish that the vehicle has one before pursuing this at all.

Atomisation is the point and the reason a restriction matters so much. Reductant delivered as a fine mist evaporates and reacts across the catalyst; delivered as a coarse stream it lands on metal that is not hot enough to evaporate it and crystallises into a hard white deposit. So a restricted air line does not merely reduce performance — it actively creates the blockage that then fails the injector.

The restriction is usually crystallised reductant itself, which makes this self-accelerating: a little poor atomisation deposits solids, which restrict further. Corrosion and a kinked line after work are the other routes. Check air pressure at the injector rather than at the pump, since the restriction lies between them, and expect to inspect the injector in the same visit — by the time the line is restricted the injector is usually already coated.