Diesel exhaust fluid is a non-hazardous solution applied to the exhaust stream of diesel vehicles to break down nitrous oxide emissions into nitrogen and water. This system, called “selective catalytic reduction,” is found on 2010 and later model diesel vehicles.
Diesel exhaust fluid is comprised of 32.5 percent urea and 67.5 percent de-ionized water. DEF is not a fuel additive and is stored in a separate container from diesel, and the two never come into contact. The fluid has a shelf life of up to two years, but will expire more quickly if it remains above 86 F for an extended period or is exposed to direct sunlight.