2.5 GALLON DEF

DEF is injected into the exhaust stream of diesel engines equipped with a Selective Catalytic Reduction (SCR) system to break down harmful nitrogen oxides (NOx) into harmless nitrogen and water vapor. Truck owners add it to the dedicated DEF tank; the fluid is consumed during normal operation and must be refilled periodically—typically every few thousand miles depending on the vehicle. Using DEF is mandatory for meeting EPA emissions standards on virtually all heavy-duty trucks built from 2010 onward, and running out can cause engine derate or prevent the engine from starting.