A catalytic converter consists of a stainless-steel casing that houses a metallic (metalith) or ceramic (monolith) substrate. This substrate has a very large number of fine channels running through it along its length.

The aim here is to create the greatest possible surface area to optimise the catalytic effect. The surface of the substrate is coated with a highly porous layer (washcoat) to which precious metals (platinum, palladium and/or rhodium) are added in suspension.

It is these precious metals that are the actual catalysts initiating the chemical reactions required to purity the exhaust gases.