Contained within the stainless steel shell of an aftermarket converter lies a substrate "brick" that is coated with a combination of platinum, palladium, and rhodium. These three elements are frequently called precious, or noble, metals.
Hot exhaust gases containing the "terrible trio" (carbon monoxide, oxides of nitrogen and hydrocarbons) of pollutants travel through the exhaust pipe and eventually come in contact with the precious metals that are loaded on the converter's substrate. The substrate is a honeycomb of small ceramic passageways.
When the exhaust gas comes in contact with the precious metals or catalyst, a chemical reaction takes place that weakens the bonds of the polluting chemicals and allows them to easily converter into the more desirable by-products of combustion.