Three-Way Catalytic Converter
The three-way catalytic (TWC) converter is used in conjunction with the conventional oxidation catalytic (COC) converter. All models with the Oxygen Sensor/Feedback Carburetor system use this type of converter. American Motors, Chrysler Corp. and Ford Motor Co. use 2 separate converters, while General Motors uses only 1 converter.
On some Ford Motor Co. models, first converter in the exhaust system is a light off catalyst (LOC) converter. This is a single-bed converter designed to control exhaust emissions during engine warm-up, when main converter(s) is not at temperature required for maximum efficiency. On all other models, first converter in exhaust system (3-way) reduces hydrocarbons (HC) and carbon monoxide (CO), but mainly oxides of nitrogen (NOx). The second converter (oxidation) reduces, with the extra air provided by the AIR pump, hydrocarbons (HC) and carbon monoxide (CO) only.