Catalytic converter. Computer illustration showing the function of a three-way catalytic converter. Catalytic converters are used on vehicles to control their emissions. They convert toxic pollutants (gold) in exhaust gas to less toxic ones (blue) by catalysing redox reactions (oxidation or reduction). Two-way catalytic converters oxidise carbon monoxide to carbon dioxide and hydrocarbons (unburnt and partially burnt fuel) to carbon dioxide and water. Three-way catalytic converters also reduce nitrogen oxides to nitrogen and oxygen.