It is not possible to meet the current type approval standards for the exhaust emission from internal combustion engines without the use of exhaust aftertreatment systems. An additional problem is in the increased emissions in real driving conditions compared to the values obtained in type approval tests. The current legislative trends are not limited to just lowering the emission limits for new vehicles, but also actions aimed at reducing the impact of vehicles that are already in operation. One of such actions is known as retrofitting, described as retrofitting older structures with modern exhaust aftertreatment systems, without the need for any changes in the engine design. Research has shown that the emission of toxic compounds from agricultural machinery engines can reach values up to 30 times higher than those of an average passenger vehicle. This problem has been noticed by the legislators and regulations concerning operated non-road machines are successively being introduced. Such restrictions are already present in the Berlin city center for example, where all vehicles operated within the given zone must meet the Stage 3b emission limits. Retrofitting is a multi-stage process. The article discusses the issue of performing this process, starting with NRMM (Non-Road Mobile Machinery) emission measurements, through simulation studies of a virtual model of a particulate filter in CFD software (Computational Fluid Dynamics), to verify its operation in an agricultural tractor, i.e. one of the most numerous vehicle types within the non-road vehicle group in Poland.