Diesel exhaust fluid is one of the key elements of the Selective Catalytic Reduction (SCR) process used by most medium and heavy-duty engine builders to meet EPA 2010 regulations. BlueDEF is a non-toxic solution of 67.5 percent purified water and 32.5 percent ultra pure automotive grade urea. BlueDEF is not a fuel or fuel additive. Instead, when injected into the exhaust stream and passed over a catalyst, BlueDEF helps convert NOx into nitrogen gas and water vapor, two harmless and natural components of the air breathe. BlueDEF is stable, colorless and odorless, and meets ISO Standards 22241 for purity and composition and is an American Petroleum Institute (API) certified diesel exhaust fluid. These are the highest quality and safety standards in place to ensure optimum SCR performance. Making sure you do not run out of BlueDEF in a SCR equipped truck is simple. Watch the Diesel Exhaust Fluid (DEF) gauge, which is part of the diesel fuel gauge, for when it is time to refill. DEF is stored in a dedicated tank next to the fuel tank on the driver's side. The tanks range in size from 6 to 23 gallons depending on the truck's application. The DEF tank fill opening is designed to accommodate a DEF fill nozzle to ensure only DEF is put into the tank. A diesel fuel nozzle will not fit into the DEF tank opening.