The purpose of any catalytic converter is to reduce harmful emissions from the exhaust of a "Properly Tuned" combustion engine. It accomplishes this through a combination of heat and precious metal catalyst that causes the harmful emissions to either oxidize or reduce to safe elements in the exhaust flow. If the engine is out of tune and not calibrated to OEM specs, the catalytic converter's efficiency is greatly diminished and cold lead to a converter failure.
The precious metal catalyst is bound to an extruded ceramic honeycomb substrate. The ceramic has hundreds of flow channels that allow the exhaust gasses to come in contact with a maximum amount of surface area where the catalyst reaction takes place. The catalyst must come in direct contact with the exhaust gasses for the reaction to take place.
If the ceramic inside you converter becomes clogged or coated with carbon, lead or oil, then the converters efficiency is greatly reduced.
There are three basic types of automotive catalytic converters: Two-Way, Three-Way and Three-Way + Air. Each type uses a slightly different method of chemistry to reduce the harmful elements in exhaust emissions. Early model converters used a pelletized catalyst but most modern converters are now designed with a free-flowing honeycomb ceramic catalyst. The type of converter required on a particular vehicle varies with model year, engine size and vehicle weight. Some vehicles even make use of more than one type of converter or a pre-converter to meet emission reduction standards.