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but were afraid to ask: 
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TR3driver
Great Pumpkin - R.I.P
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It has changed, unfortunately. Kind of the same problem we have here, too many people with too many cars (trucks, buses, lawnmowers, fireplaces, cows, etc)
https://patch.com/colorado/denver/denvers-air-quality-among-worst-nation-report-finds
Ex post facto refers to a law that takes effect before it is passed; current smog testing does not meet that definition. So they could not, for example, say now that a car you bought last year had to meet a standard at that time. But saying now that to drive a car on public roads next year, it has to meet certain standards, is not ex-post facto. They could even (within the constraints of "no ex post facto" anyway) forbid older cars on public roads unless they were modified to meet more recent standards.
I've not been through Colorado testing, but here in CA, the test limits are tailored to the expected emissions when the car was new, with a generous allowance for normal wear. If you flunk the test, there really is something wrong, even if there are no other symptoms. For example, the first time my motorhome got tested, it flunked pretty bad. Turned out I had a bad plug wire but hadn't noticed it. New set of wires (and some carb cleaning) and it passed. Another time, my Chevy flunked, and it turned out there were no pellets left in the catalytic converter! You could literally see through it (from intake to exhaust).
For some time here, all old cars had to be tested when title was transferred (similar to Colorado I believe). I was fairly panicked about getting my previous TR3A through smog, as I knew it had cylinder wall damage (and had never been through a smog test before). Tuned it as clean as I knew how: fresh oil, new points and plugs, mixture as lean as it would run without misfiring, timing on the late side and so on. It flunked the test, because the exhaust was too clean! Seriously, their machine said it must have an exhaust leak to be that clean. Perhaps it did; but all I did was retune it "by the book" and it passed with flying colors.
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