DEF (Diesel Exhaust Fluid)

DEF is Diesel Exhaust Fluid, a mixture of urea and deionized water that your truck injects into the exhaust stream. It works with the SCR catalyst to convert harmful nitrogen oxides into harmless nitrogen and water.

A typical heavy duty truck uses DEF at about 2 to 3 percent of its diesel consumption. The truck warns you when the tank runs low and will eventually limit speed if you ignore it.

DEF goes bad with age and heat. Old or contaminated DEF triggers reductant quality fault codes. Always use fresh certified fluid from a sealed container.

Mike Turner

Red Seal certified heavy duty mechanic with 14 years turning wrenches on Cummins, Powerstroke and Duramax engines across Alberta and Ontario. Mike writes the guides he wishes existed when he was apprenticing.