Selective Catalyst Reduction (SCR Or Adblue System) The last component in the aftertreatment system is a flow through Selective Catalyst Reduction (SCR) catalyst which introduces Diesel Emissions Fluid (DEF) to the process. Diesel exhaust fluid is an aqueous urea solution made with 32.5% urea and 67.5% deionized water This fluid contributes to the further break down of nitrogen oxides that pass through to the SCR filter
