An SCR (Selective Catalytic Reduction) system is a system installed on diesel vehicles to reduce harmful Nitrous Oxide (NOx) emissions. It works by injecting an automotive grade urea, or Diesel Exhaust Fluid (DEF) through a specially designed catalyst, into the exhaust stream of a diesel engine.

The Diesel Exhaust Fluid sets off a chemical reaction which converts the NOx into Nitrogen, water and very small amounts of CO2. SCR technology can achieve up to 90% NOx reduction.

The technology has been around for some time, and used extensively on commercial on and off-road vehicles. In addition, cargo vessels, ferries and tug boats worldwide have employed SCR technology to effectively reduce emissions.

One of the largest sectors for SCR technology is in the US. 2010 saw the introduction of the EPA diesel engine emissions standards for heavy duty commercial trucks. SCR had the added benefit of some commercial truck operators reporting 3-5% increase in fuel economy. Such is the importance of Diesel Exhaust Fluid to the emissions control systems, many vehicle starting systems may be locked out if inadequate levels of Diesel Exhaust Fluid are onboard, preventing the vehicle from starting.