The purpose of this study was to perform both experimental and computational investigations on the selective catalytic reduction (SCR) system in passenger cars equipped with compression ignition (CI) engines. The study involves a comparison of results obtained for two separate SCR systems: an existing one and a newly developed system. The newly designed SCR system is intended for implementation in the spare parts market (Aftermarket) and includes the creation of a custom mixer design. This research analyzed multiple SCR systems and mixers under varying operating conditions. Various factors were considered, including the examination and evaluation of gas distribution and nitrogen oxide reduction. The multiphase computational fluid dynamics analyses were conducted using the ANSYS Fluent software. A detailed assessment was carried out for the sequential processes occurring within the system. The final version of the replacement SCR system was analyzed in relation to the original system supplied by the original equipment manufacturer (OEM). The implementation of the new mixer in the replacement SCR system led to slightly reduced NOX emissions, as validated by emission tests (NEDC) performed in a car on a chassis dynamometer within a certification unit.