DEF is urea that helps clean the exhaust before it's released from your vehicle. DEF stands for diesel exhaust fluid just in case you were wondering. If your vehicle has the system on it, then it's required for the engine to run correctly. There is a difference between the three systems and the type of fluid so if you're refilling it on your own just make sure you have the right stuff for it.
2012 was the last year that it wasn't federally mandated that you had to have a DEF system on your diesel vehicle. Ford and Chevy both went to the DEF systems before it was mandated and Dodge switched in 2013.
If you have questions give us a call at 541-791-2886 and we would be happy to help out.