Crystal Blue is used in vehicles equipped with a Selective Catalytic Reduction (SCR) system, which helps treat exhaust gases before they exit the vehicle. Many heavy-duty vehicles, especially those meeting Euro 4, Euro 5, and Euro 6 emission standards, rely on an SCR system and thus use Crystal Blue. This includes most trucks manufactured after 2005, and the first diesel cars with SCR systems were introduced in 2014 to meet Euro 6 standards.

The SCR system is located behind the particulate filter in the exhaust. When NOx levels in the exhaust reach a certain point, a precise amount of Crystal Blue is injected from the reservoir. Inside the SCR, Crystal Blue is converted into ammonia and a small amount of carbon dioxide. The ammonia then reacts with the NOx, turning it into harmless nitrogen and water. The SCR catalyst ensures that this reaction happens quickly and effectively.