SCR

📖 YOUCANIC Automotive Glossary

SCR (Selective Catalytic Reduction) is the emissions aftertreatment system on modern diesel vehicles that converts nitrogen oxide (NOx) emissions into harmless nitrogen gas and water vapor. The system works by injecting a precise amount of AdBlue/DEF upstream of the SCR catalyst — inside the catalyst, the urea breaks down into ammonia, which reacts with NOx in the presence of the catalyst substrate. The ECU controls DEF injection rate based on upstream NOx sensor readings, exhaust temperature, and exhaust flow rate.

SCR system components include the DEF tank, DEF pump, DEF injector, SCR catalyst, upstream and downstream NOx sensors, and exhaust temperature sensors. Common DTCs: P20E8 (Reductant Pressure Too Low), P2BAA-P2BAF (SCR efficiency codes), P229F (NOx Exceeds Limit). The YOUCANIC UCAN-II provides SCR system diagnostics including DEF injection rate monitoring, SCR catalyst efficiency data, and system reset functions. SCR problems are often caused by DEF quality issues, injector crystallization, or NOx sensor failures rather than the catalyst itself.