Title 2 of the Clean Air Act "authorizes the EPA to set standards applicable to emissions... the CAA prohibits tampering with emissions controls, as well as manufacturing, selling, and installing aftermarket devices intended to defeat those controls."

They just got a $10M civil judgement against a couple "diesel tuners" here in Michigan:

https://www.justice.gov/opa/pr/united-states-awards-10-milli...

https://www.epa.gov/enforcement/national-enforcement-and-com...

but yeah, this is civil action against vendors, not anything that police will fine people for on the side of the road.