What is Diesel Exhaust Fluid or (DEF)?   Diesel Exhaust Fluid (DEF) is required for diesel engines and helps to control emissions. DEF is a non-hazardous solution that is mostly water. DEF contains 67.5% de-ionized water and the remaining percentage is urea. In fact, it looks just like water. DEF smells a bit of ammonia or like the cleaning supplies you probably have at home. ...