Diesel exhaust fluid is an engine fluid used in selective catalytic reduction systems to help cut down on the nitrous oxide emissions of diesel engines. The heat generated by the DEF allows the selective catalytic reduction system to break down the nitrous oxide into innocuous elements.
Nitrous oxide emissions contribute to global warming and acid rain. As of 2010, the Environmental Protection Agency has issued new emissions standards and dictated that all on-road diesel vehicles made after 2010 must be equipped with exhaust systems to reduce nitrous oxide emissions. Selective catalytic reduction systems that use DEF help reduce nitrous oxide emissions in these vehicles between 70 and 90 percent.