Contrasting that to California, I had an old well-maintained Volvo station wagon. California tightened the emissions standards on that model until it failed smog (at 110% of NOX limit), and made me upgrade the catalytic to an after market one. It then passed at 85% the NOX limit.

The state strongly encouraged me to send it to a Golden Shield mechanic. As part of the work, they "checked" the air intake filter and MAF sensor on the engine, which was, of course, filthy. It sits in a big plastic box on that car. Instead of reinstalling the filter, they jammed it in sideways and closed the box. I figured it out later, and spent about an hour picking bits of leaves out of the downstream air intake.

Of course, this cost 95% of the limit past which California gives you a free pass till the next smog inspection. (And almost 2x the market value of the car.)

I'm convinced the whole thing is a make-work program that's holding our air quality hostage. (And I'm strongly for tightening emissions standards!)

Many states barely even check anything anyway. Out of six states I've had a vehicle inspection in, only one (Maryland) did more than check lights, wipers, brakes, and horn. Inspections seem to be more a revenue stream than anything else.

How do you tell if there is no vehicle failing because your inspection takes them off the roads or if it's because nobody wants them anyway?

Any kind of policing is self-denying. You can't evaluate it while it's being applied.