The catalytic converter, commonly known as the catalyst, is the most important element of the exhaust system responsible for exhaust gas cleaning, and together with the DPF filter, it is the exhaust gas cleaning system in automobiles.

Gasoline engines use three-way reactors, including the oxidation of hydrocarbons to water and carbon dioxide and the reduction of nitrogen oxides. Diesel vehicles use oxidation reactors, which oxidize carbon monoxide molecules and hydrocarbons. Catalysts are made of a ceramic monolith with a honeycomb structure or metal (made of a perforated tape). The entire catalyst is usually in the form of a long metal tube of various diameters and shapes.