Catalytic Converters were required starting with the 1975 model year in the U.S.

 begin quotes:

 

Catalytic converters became mandatory starting with the 1975 model year in the United States, driven by the U.S. Environmental Protection Agency and the Clean Air Act of 1970. 

Key Timeline and Rules

1970: Congress passed the Clean Air Act, demanding a 90% cut in vehicle pollution.
1975: Car makers added catalytic converters to almost all new gas cars in America to meet the new low pollution limits.
1993: Countries in the European Union made these devices mandatory for all new gas cars starting January 1, 1993. 

If you want to know more, let me know if you need info on:

State-specific rules (like California's laws)
How the device works to clean exhaust

• Catalytic converter - Wikipedia

  ... from a vehicle, except in order to replace it with another converter, and Section 203(a)(3)(B) makes it illegal for any person...

  

  

  Wikipedia
• When Did Cars Start Using Catalytic Converters? - Jalopnik

  The '50s and '60s had been fun for the American auto industry, but the fun had to stop. At President Richard Nixon's request, in 1...

  

  

  Jalopnik

Show all