The short answer
Whether you need a smog check depends on one thing: where the car is registered. Right now 24 states run some kind of emissions program. The rest have none, so a driver in Michigan or Florida never tests, while a driver in Los Angeles tests every other year. County-level rules add a wrinkle in states like Texas, Illinois, and Nevada, where the requirement follows your county, not the whole state.
Cars that usually skip the test
Even in testing states, a few groups get exemptions most of the time. Brand-new cars typically get a pass for their first few registration years. Diesel vehicles, motorcycles, and trailers are excluded in many programs. And cars old enough to be classed as antiques or classics often qualify for historic plates that skip testing. The cutoffs differ by state. That is exactly what the tool above links to, so confirm on your state's page before assuming you are exempt.
What happens if you skip a required test
In states where testing is tied to registration, the DMV simply won't renew your tags without a passing certificate. There's usually no fine or penalty beyond that; you just can't legally drive on expired registration. In California, a car that repeatedly fails can get flagged to a STAR station for its next test, which is worth knowing before you shop around for the cheapest option.
Frequently asked questions
I just moved to a testing state. Do I test right away?
Usually yes, at your first registration in the new state. Some states give you a short window after establishing residency. Bring your out-of-state title and registration; the testing station handles the certificate the DMV needs.
Does an electric car need a smog check?
No. Battery-electric vehicles have no tailpipe emissions and are exempt everywhere in the U.S. Hybrids are a different story: most states still test them because they burn gasoline.
How do I find out my county's rule?
Enter your ZIP above, open your state page, and check the county list there. County rules change now and then as air quality targets shift, so the state page is the place we keep current.
Related: smog check finder & cost estimator, what a smog check actually inspects, price guide by state.