they remove your gascap, it will throw a CEL due to lack of pressure in the tank...

they run your car and do the various tests all at once... but it's been a couple of years since i've done one - so maybe there's a new process?

'04 Mazda6S MTX Injen CAI, 18% Tint, Magnaflow Catback, CPE-MAFc My Mazda6 PICS Toadster's Website