What is the purpose of a Diesel Exhaust Fluid (DEF) system in modern diesel engines, and what happens if the system malfunctions or DEF is contaminated?

The purpose of a Diesel Exhaust Fluid (DEF) system in modern diesel engines is to reduce emissions of nitrogen oxides (NOx). NOx are harmful pollutants that contribute to smog and acid rain. The DEF system uses a process called Selective Catalytic Reduction (SCR) to convert NOx into harmless nitrogen and water. The DEF, which is a solution of urea in water, is injected into the exhaust stream upstream of the SCR catalyst. The heat from the exhaust causes the urea to decompose into ammonia. The ammonia then reacts with the NOx on the surface of the....