The emissions test takes place on a dynamometer, which would result in a large number of weird things the car could test for.
Such as open doors/hood while the wheels are spinning, no acceleration or a stationary GPS signal.
The fact that cars have GPSs, make it next to impossible to design tests that can't be cheated, you would need a testing rig that could be mounted on a moving car, that didn't cause an increase of pressure on the exhaust system (cars could detect that) And then you would need to do testing on public roads, so the car didn't switch to a different mode when it's GPS showed it was on a known test track.
Volkswagen used a ECM that sensed if the vehicle was being tested based on various inputs including the position of the steering wheel, vehicle speed, the duration of the engine's operation, and barometric pressure.
(source EPA.gov)
In my state, PA, they use an inductive pickup on the spark plug wire for cylinder #1 to determine the RPMs and a probe that's inserted into the tailpipe. The probe is a small diameter (0.5" or less) tube and it shouldn't raise the back pressure by any appreciable amount.
Maybe an EE could tell us if it's possible for the ECM to detect the presence of the inductive pickup.
EE here. Technically it's possible to detect an inductive pickup. But they are pretty subtle. You would probably need a very specially designed coilpack.
You can't, but that doesn't really affect the cheat. I mean you could do: if gps not available default to clean mode. That would benefit you during testing or when you're in an underground garage or tunnel (Places where you would like to have as clean exhaust as possible anyway).
There is talk of emissions controls being linked to GPS location so that each country/state/city might enforce different emissions rules. Some high-end Japanese machines already use GPS to determine whether or not they are on a racetrack, opening up performance and speeds normally locked away while on public roads.
It is very practical - the whole Volkswagen scandal is caused by the fact that the compromise of low emissions vs better performance and mileage can be dynamically changed by software, and that a car that can conform to the tightest standard still would prefer use a mode with higher emissions whenever and wherever allowed.
It would enable your car to suddenly perform better as soon as you enter a state with less strict emission requirements, or vice-versa, your usually high emission car would be legal to operate when visiting stricter places because it can switch to a limited more when needed.
Such as open doors/hood while the wheels are spinning, no acceleration or a stationary GPS signal.
The fact that cars have GPSs, make it next to impossible to design tests that can't be cheated, you would need a testing rig that could be mounted on a moving car, that didn't cause an increase of pressure on the exhaust system (cars could detect that) And then you would need to do testing on public roads, so the car didn't switch to a different mode when it's GPS showed it was on a known test track.