I have to dispute that comment. EPA does follow-up testing on in-use vehicles to make sure the emission controls are still working properly. When I ran a municipal fleet, they contacted us a couple of times to get vehicles to test. This quote is from a story about Cummins recalling 500k medium/heavy truck engines in 2018"

“Today’s recall is a great example of how government and industry work together to protect health and the environment." said EPA Office of Air and Radiation Assistant Administrator Bill Wehrum. “This is the way it’s supposed to work. Our follow-up testing seeks to make sure that pollution controls work throughout an engine’s useful life. And, if they don’t, then companies step up to set things right.”

This is how VW got caught cheating on diesel emissions.