SCR (Selective Catalytic Reduction)

Selective catalytic reduction is the most effective way to reduce NOx from modern diesels. AdBlue sprayed into the exhaust turns into ammonia, which reacts with NOx on the catalyst surface. The control unit checks efficiency with NOx sensors before and after the catalyst. If efficiency is too low, it sets a code and starts a countdown after which the engine may not restart.

SCR is widely used on modern diesel cars, vans and trucks to meet Euro 6 and later NOx limits. SCR systems are monitored by NOx sensors before and after the catalyst. If the reduction is too low, the control unit stores a fault and, depending on the market and vehicle, may limit power or block restarts. Many vehicles combine SCR with a diesel oxidation catalyst and a particulate filter in one exhaust module.

The dosing amount follows the engine’s NOx output and exhaust temperature. The system needs heat to work and protection against freezing. AdBlue is injected into the hot exhaust, where it turns into ammonia. In the SCR catalyst, ammonia reacts with nitrogen oxides to form nitrogen and water.

Common faults are crystallised AdBlue, failed dosing pumps or heaters and NOx sensors, with codes such as P20EE or P20B9. Poor-quality fluid can damage the system. SCR faults usually come from the AdBlue pump, heater, injector, NOx sensors or poor-quality fluid. Warnings often count down the distance remaining before the engine will not restart.