What Is A Catalytic Converter And Why Do You Need One? Some petrolheads see them as a nuisance, but cats are a vital component for your vehicle's emissions control. What is a catalytic converter and what does it do? The cat sits around a third of the way down the exhaust system and resembles a small metal chamber that receives exhaust gasses and changes the chemical nature of them to reduce the volume of nasty emissions fresh from the exhaust manifold. Within the cat housing is a ceramic-based honeycomb structure that is lined with extremely precious metals, with each metal having a specific job in emission-reduction. There are three main emissions produced by car engines: nitrogen gas (N2), carbon dioxide (CO2) and water vapour (H2O). The catalytic converter however is mostly used to tackle the smaller, more-harmful products that are produced due to the naturally-imperfect combustion process of the IC engine. These are carbon monoxide, hydrocarbons and nitrogen oxides.