[alirazaq]

About the same in California; mandatory emissions check every 2-years. In the case of a private sale, if the owner has not had it inspected within 90-days of sale the buyer is required to have it inspected upon registration.

There are exceptions with older (vintage) cars, and relatively new cars.

As with most things, these laws vary from state-to-state, and even county-to-county within said states.

[panglott]

California is known for having the toughest regulations in this area.

Kentucky had a limited emissions inspection regime from 1999-2005, when it was killed by the Republican governor. The only current inspection requirement I know of is if you purchase an out-of-state used vehicle. We have plenty of ancient cars on the road that have never been inspected.

Sometimes police officers will pull you over if you have obviously-broken headlights or such, and tell you to fix it, but not often.

[maxerickson]

Wikipedia has a decent article about where in the US emissions testing is required:

en.wikipedia.org/wiki/Vehicle_inspection_in_the_United_States