For cars 1996 and after, it's only obd2 plug... they do some scanning, make sure there are no faults and all the Emission stuff is operational.
We use to have that Gateway Clean Air program that acted like a Dyno with plugs for exhaust... however after number of engines getting damaged by people forgetting to turn the fans on or something, they canceled the program.
Cars before 1996 only require safety, cars after do the emissions and safety... but it's only that obd2 plug check thing.
2013 Audi //RS5 Phantom Black Current
2009 e92 ///M3 Jerez Black<~~Sold
2009 Audi //S5 Phantom Black<~~Sold
2002 e46 ///M3 Carbon Black <~~Sold
2007 Audi //RS4 Dolphin Gray<~~Sold