SCR (Selective Catalytic Reduction)

SCR stands for Selective Catalytic Reduction. It is the section of the exhaust system, downstream of the DPF, where DEF is injected and a catalyst converts nitrogen oxides into nitrogen and water vapor.

When the SCR system reports low efficiency, the usual suspects are bad DEF, a failed NOx sensor, or a worn catalyst, in that order. Diagnose in that order too.

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.