<P> 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 . </P>
<P> 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 . </P>
when did catalytic converters become mandatory in us
when did catalytic converters become mandatory in us
1975
1975