Automotive industry is one of the largest industries all over the world. There are worldwide companies and individual local automotive companies. There are restricted regulations and standards related to automotive industry including emissions. Reduction of emissions is a very important problem. Alternative solutions are offered to reduce the emissions. In this study, reduction of the emissions is numerically investigated for different channel geometry, inlet temperature, and wall temperature of the catalytic converter channel. It was found that increasing the wall temperature activates the catalytic converter and produces more reduced emissions. The channel geometry has little effect on the conversion efficiency of the catalytic converter due to the same hydraulic diameters of the channels. In conclusion, the cold-start engine condition preheating could be applied to the exhaust gas for more environmentally friendly operation.