In most or all counties its once every 12 months, usually with a 15 to 30 day grace period after that
once a year
I never replace it ,only if you need to like before a smog test so it can pass emissions
your vehicle must have an emissions test for registration renewal, beginning when it is five years old. If your vehicle requires an emissions test, you will receive a reminder as part of your vehicle licence renewal application
every 2 years
at least 20 to 21 years
If it is leaking or rotted
About 60 miles in order for the computer to log emissions information to relay to the testing computer.
more than likely if your state requires emissions testing.
In the state of CT in the USA, if the vehicle is 25 years old or older it does not have to go through an emissions test.
If it came with one from the factory, yes if you want to pass emissions test.
Not every county in Virginia require emission inspection but the counties that do it is every two years.
Not if they test emissions.