Diesel Exhaust Fluid (DEF) is a mixture of high-purity urea and deionized water. It is sprayed into the exhaust before the catalytic converter. Heat turns the urea into ammonia. This ammonia reacts with nitrogen oxides (NOx), converting them into harmless nitrogen and water vapor. DEF is stored in a separate tank and must be refilled regularly.
Running out of DEF will put the vehicle into a slow limp mode. The engine will not restart once turned off if the tank is completely empty. Always keep a spare jug of DEF if you plan a long road trip in a diesel truck.