For many vehicles with Euro 4, 5 and 6b certification the real-world on-road emissions (Real Driving Emissions – RDE) are higher compared to their certified values. With the latest EURO steps (Euro 6d-temp and Euro-6d) vehicles are compliant to the emission limits also when driving on the road. But older vehicles still emit much higher NOx emissions (diesel) and particulates (petrol). To prevent that these older vehicles are banned from the center of cities across Europe and to maintain urban air quality, an option is to equip them with Retrofit Emissions Control (REC) systems

Older Diesel vehicles with an emission standard of EURO5 or older contribute to the overall NOx emissions, caused by Diesel powered vehicles, by about 78%, EURO6a,b and c vehicles by about 21% and only 1% by EURO6d-Temp and EURO6d. One EURO4 vehicle emissions are as average about 21 times higher than for a EURO6d-Temp vehicle.