In the Washington DC metro area, all local juruisdictions are required to smog-test vehicles every two years. Here in Fairfax-County the emissions inspection covers all gas vehicles newer than 25 years-old, and below a certain weight (7500 pounds I think), but only diesels, '96 and newer, with the OBD-II engine computer.
So far, older diesel cars and light trucks in the US haven't been a big target because, unlike Europe, they make up a relatively small percentage of the vehicles on the road.
For now I guess, that's been one advantage to diesels being relatively unpopular, for so long here.

Happy Motoring, Mark

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DrDKW