Catalytic converters are used to clean engine exhaust emissions. Your Fairmont or Zephyr may be equipped with one or two converters. Models having only one converter in the exhaust system, have the Conventional Oxidation Catalyst (COC) converter. The COC acts on two of the major pollutants: unburned hydrocarbons (HC) and carbon monoxide (CO). If the exhaust system has two converters, the first (and usually smaller) converter, called a Three-Way Catalyst (TWC), is designed to control oxides of nitrogen (NOx).
The TWC converter operates on the exhaust gases as they arrive from the engine. As the gases flow from the TWC to the COC converter, they mix with air from the air pump injected into a mixing chamber. This air is required for proper oxidation in the COC converter.
The continued use of leaded gas in a converter equipped car will clog the system and render the converter useless.