The paper presents a comparison of the results of exhaust gas emission measurements from Euro 3 and Euro 4 vehicles equipped with catalytic converters with the results of exhaust gas emission measurements from Euro 3 and Euro 4 reference vehicles with catalytic converters removed. The main objective is to show the impact of removing catalytic converters from Euro 3 and Euro 4 emission standard vehicles. It has a negative impact on the increase of carbon monoxide (CO) emissions into the environment, as the most toxic gas produced in the process of fuel combustion in the engine. The obtained result showed that the removal of the catalyst, whether it is an engine with Euro 3 or Euro 4 emission standard, there is a significant increase in the emission of carbon monoxide (CO). The exhaust gases, which do not exceed the legal limit when the engine is idling, exceed the legal limit at higher engine speeds. The characteristics of the vehicle were also analyzed to determine what other factors influence the increased CO emissions. Through this analysis, it was determined that the number of kilometers driven significantly affects the reduction in catalyst functionality.