SCR Efficiency

📖 YOUCANIC Automotive Glossary

SCR Efficiency is the measured percentage of nitrogen oxide (NOx) emissions that the SCR (Selective Catalytic Reduction) catalyst successfully converts into harmless nitrogen and water vapor. The ECU calculates efficiency by comparing upstream NOx sensor readings (before the SCR catalyst) to downstream NOx sensor readings (after the catalyst) — for example, if the upstream sensor reads 400 ppm NOx and the downstream reads 40 ppm, the SCR efficiency is 90%. EPA regulations require SCR efficiency above a manufacturer-calibrated threshold (typically 85-95% depending on conditions) to maintain emissions compliance.

When SCR efficiency drops below the threshold, the ECU stores DTCs (P20EE, P229F, and related codes) and initiates the inducement/derate process that progressively reduces engine power. Low SCR efficiency can be caused by degraded catalyst substrate, insufficient or incorrect DEF dosing (injector problems, pump problems, empty tank), poor-quality or contaminated DEF, NOx sensor inaccuracies giving false readings, and exhaust temperatures too low for the SCR reaction to occur efficiently. The YOUCANIC UCAN-II displays SCR efficiency in live data along with upstream/downstream NOx values, DEF injection rate, and catalyst temperature for comprehensive aftertreatment diagnosis.