Diesel Exhaust Fluid (DEF)
A non-toxic solution of high-purity urea and deionized water injected into the exhaust stream of diesel engines to break down harmful nitrogen oxide emissions.
Diesel Exhaust Fluid (DEF) is a crucial consumable fluid used in modern diesel vehicles equipped with Selective Catalytic Reduction (SCR) technology. DEF consists of a precise mixture of 32.5% high-purity synthetic urea and 67.5% deionized water. It is stored in a dedicated onboard tank and is sprayed in tiny, metered amounts directly into the hot exhaust gases upstream of the SCR catalyst.
When heated in the exhaust pipe, the urea in DEF vaporizes into ammonia and carbon dioxide. Inside the catalyst, the ammonia reacts with harmful nitrogen oxides (NOx) produced by the diesel engine, breaking them down into harmless nitrogen gas and water vapor, significantly reducing tailpipe emissions.
Vehicles track DEF levels closely and will warn the driver when the tank is low. If a diesel vehicle runs completely out of DEF, the engine computer will enter a restricted start or "limp" mode, limiting vehicle speed or preventing the engine from restarting until fluid is added. It is critical to use clean, uncontaminated DEF to avoid damaging the delicate SCR system.