Diesel Exhaust Fluid (DEF) is a solution of urea and water that is injected into the exhaust stream of diesel vehicles to turn NOx gases (harmful emissions) into nitrogen and water. DEF is not a fuel additive and is stored in separate tanks. DEF is a requirement for medium- and heavy-duty vehicles with diesel engines produced after 2010. The vehicle is programmed to inject the DEF into the exhaust stream to meet emissions requirements. DEF is made from 32.5% urea and 67.5% de-ionized water. DEF is a clear liquid that can be bought at gas stations, supermarkets, auto parts stores, or even big box home improvement stores, and it usually comes in 1-gallon jugs or 2.5-gallon cardboard boxes with a plastic liner inside. DEF is injected into the exhaust stream of diesel vehicles to reduce NOx and air pollution, helping heavy machinery and trucks operate within the federal emissions guidelines. DEF is part of the Selective Catalytic Reduction (SCR) system, which drastically reduces the amount of pollution emitted from vehicles. DEF is sprayed into the exhaust system to break down the generated NOx into harmless nitrogen and water molecules. The DEF tank is often located right next to the diesel tank, and the DEF tank has a blue cap, while the diesel tank has a green cap. If a vehicle runs out of DEF, the engine performance will be reduced, and lower speeds are imposed. Over the long run, repeatedly running out of DEF can also cause the vehicles exhaust system to clog with soot and debris, leading it to run less efficiently and eventually break down.