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• $40.90 for a smog check for one car ($79 value)

Catalytic Converters: Keeping Combustion Clean

Cleaner cars make for cleaner air. Learn how catalytic converters help with Groupon’s guide.

A car’s engine is a dirty place, and it produces even dirtier exhaust. Vapors exit the combustion chamber carrying a nasty trio of smog promoters: nitrogen oxides, hydrocarbons, and carbon monoxide. As these pollutants barrel through the exhaust manifold (the set of tubes that feeds exhaust from each cylinder into the exhaust pipe), there's only one thing standing between them and the innocent air outside: the three-way catalytic converter.

Situated just before the muffler, the converter is home to honeycombed ceramic cores coated with some combination of platinum, rhodium, and palladium—three rare metals that interact with the main components of the pollutants. When exhaust enters, the metals trigger chemical reactions that heat the gasses and rearrange their molecules. Under their influence, nitrogen oxides splinter into pure oxygen and nitrogen, while hydrocarbons and carbon monoxide are oxidized into less-harmful carbon dioxide and simple water. Since their introduction during the 1975 model year, these unseen cleaners have made air more breathable and much tastier: the EPA estimates that cars built today are 98% cleaner than their counterparts from the 1960s.