The AdBlue injector (dosing module) is part of the selective catalytic reduction (SCR) emissions system on diesel engines. Mounted directly on the exhaust pipe before the catalyst, this solenoid valve sprays a fine mist of diesel exhaust fluid (DEF) into the hot exhaust gases. The heat breaks the urea fluid down into ammonia, which reacts with harmful nitrogen oxides inside the catalyst, turning them into nitrogen and water vapor.
These injectors clog frequently due to crystallization. Because AdBlue is a mixture of water and urea, hot exhaust heat evaporates the water, leaving behind crusty white urea crystals. These crystals block the injector tip, stopping the spray and triggering an engine warning light. The vehicle's computer will eventually prevent the engine from starting if the injector is not cleaned or replaced.