DEF Injector
An electronically controlled valve that sprays Diesel Exhaust Fluid into the exhaust stream to reduce emissions.
The Diesel Exhaust Fluid (DEF) injector is an electronically controlled dosing valve mounted in the exhaust system of modern diesel vehicles. It is a critical component of the Selective Catalytic Reduction (SCR) emissions control system. The DEF injector sprays a precise amount of liquid DEF (a mixture of high-purity urea and deionized water) directly into the exhaust stream ahead of the SCR catalyst.
The heat of the exhaust vaporizes the DEF, converting it into ammonia. This ammonia reacts with toxic nitrogen oxides (NOx) inside the SCR catalyst, converting them into harmless nitrogen and water vapor.
Because DEF contains urea, it is prone to crystallizing and clogging the injector tip, particularly if the vehicle is driven on short trips where the exhaust does not reach operating temperature. A clogged DEF injector will trigger a check engine light, reduce engine power, and eventually prevent the engine from starting.