And it can vary within the states. Texas only requires emissions testing in the most populous counties (no point in testing cars in Brewster county with less th

And it can vary within the states. Texas only requires emissions testing in the most populous counties (no point in testing cars in Brewster county with less than 1 person per square mile population density). In the counties with testing, new cars get a 2-year emissions exemption, after which they get tested yearly. Testing is generally done via the OBD-II port, so the vehicle self-reports it's condition. Vehicles without OBD-II and "other" vehicles use a tailpipe probe to report on exhaust gases.