Diesel exhaust fluid (DEF) is a clear, non-hazardous, and non-toxic liquid that is used in diesel engines to reduce harmful emissions. DEF is a mixture of urea and deionized water and is injected into the exhaust stream of diesel engines to break down nitrogen oxide (NOx) into harmless nitrogen and water vapor. DEF is essential for meeting the stringent emission standards set by the Environmental Protection Agency (EPA) and is required for diesel engines equipped with selective catalytic reduction (SCR) systems.