Diesel Exhaust Fluid (DEF)
A urea solution injected into the exhaust of modern diesel engines to reduce nitrogen oxides. Consumable, and a vehicle will derate or refuse to start without it.
Diesel exhaust fluid feeds a selective catalytic reduction system that converts nitrogen oxides in the exhaust. It is consumed as the vehicle runs and refilled like a fluid rather than serviced like a part.
The operational consequence is that DEF is not optional in the way other fluids are. Run the tank empty and the vehicle will progressively reduce power and eventually will not restart, by design. For a fleet that means DEF belongs in the fueling routine and in the yard stock rather than being treated as a shop item, and an out-of-DEF plow in a storm is an avoidable failure.
It also has a shelf life and is sensitive to contamination and to freezing, so how it is stored matters more than its cost suggests.
