The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the U.S. Environmental Protection Agency 's stricter regulation of exhaust emissions, most gasoline - powered vehicles starting with the 1975 model year must be equipped with catalytic converters. These "two - way '' converters combine oxygen with carbon monoxide (CO) and unburned hydrocarbons (HC) to produce carbon dioxide (CO) and water (H O). In 1981, two - way catalytic converters were rendered obsolete by "three - way '' converters that also reduce oxides of nitrogen (NOx); however, two - way converters are still used for lean - burn engines. This is because three - way - converters require either rich or stoichiometric combustion to successfully reduce NOx.
when did catalytic converters become mandatory in us
1975