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Diesel exhaust fluid (DEF) is a non-toxic solution of 32.5% urea in de-ionized water. This fluid acts as a catalyst when introduced into the exhaust stream of diesel engines to cause to break down dangerous NOx emissions into harmless nitrogen and water. This system was added by diesel engine manufactures to meet more stringent diesel exhaust standards imposed by the EPA. The catalyst system, also called Selective Catalytic Reduction (SCR), has been in use since 2010 on heavy duty trucks and many diesel pickups and SUVs.
DEF is not a fuel additive and never comes into contact with diesel. It is stored in a separate tank, typically with a blue filler cap.
The EPA requires vehicle manufacturers ensure DEF equipped engines cannot run without DEF. Before a vehicles DEF tank runs empty the driver is alerted on his dashboard displays (much the same way as if they were running low on fuel or other fluids). For example, when the DEF tank level drops below 10% an amber warning lamp will come on, at 5% this lamp starts flashing and below 2.5% a solid amber warning light is displayed.