plenty of states with them just make sure your “check engine” light works, then does plugs in and does a scan to make sure the readiness monitors are OK and there are no codes.*
states like Texas may only do a “two speed idle” test while the vehicle is parked.
So a violating VW would be OK in 1) for obvious reasons, in 2) because the PCM would report falsely that everything was in-spec, and probably 3) because the car could stay in “compliant mode” while not moving.
I haven’t looked through all of them, but IIRC states which do use rolls for emissions inspection only do so for pre-OBDII cars.
“readiness monitors” are PCM functions which indicate the car has been driven for a certain period of time with no fault codes being set. After you clear codes- either via scan tool or a battery pull- the PCM will indicate “Not ready for test” until a long enough drive cycle has been performed.
In theory, an OBD-II scan should be sufficient to indicate compliance; modern engine controls have enough monitoring and control feedback to know whether they’re clean or not. All bets are off when the PCM code is written to intentionally obfuscate things.
All the stuff said above, plus the big fact that state allowable emissions are generally way, way more forgiving than new car EPA allowable emissions. The state maximums allow for old cars, slightly (or very) out of tune cars, cars with lots of miles, cars that when new polluted way, way more than new cars. You’re also assuming that VW “turned off smog controls” when not in EPA testing mode, when I think it’s more likely that it just tilted the scale toward performance/economy and away from emission control (that is, it didn’t turn all emissions controls off, it just dialed certain things back).
The EPA tests are lab-grade testing…very precise equipment and very precise tuned environments and situations. The local gas station with old equipment on a probe stuck in the tailpipe while a car (which might be new or might be 15 years old) idles for a few minutes is something quite different.
Some states may have sliding scales for newer car allowable vs. older cars, they’re just all over the map. But I don’t think VW forsaw a problem with state emissions testing…I think state testing in general is looking for gross polluters and really poorly maintained cars. The stuff that fails a state emissions test would be light years off the scale in the EPA lab.