SCR is an active emissions control system where hot exhaust gases flow out of the engine and into the SCR system where aqueous urea (known as Diesel Exhaust Fluid, or DEF) is sprayed onto a special catalyst. The DEF sets off a chemical reaction in the exhaust on a special catalyst that converts nitrogen oxides into nitrogen, water, and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe. The Diesel Exhaust Fluid, or DEF injector plays a crucial role in this conversion process and can malfunction due to contaminants, crystallization, extreme cold, and over dilution of DEF fluid.