Catalytic Converter: Description and Operation

The three-way catalytic converter, together with the closed loop air-fuel ratio control (based on the oxygen sensor signal) oxidizes carbon monoxides (CO) and hydrocarbons (HC), and reduces nitrogen oxides (NOx).
When the mixture is controlled at stoichiometric air-fuel ratio, the three-way catalytic converter provides the highest purification against CO, HC, and NOx.