When you take it to get an ODB-II check, those o2 sensors that are turned off will show "not ready" and that'll fail the test. They have to be turned on in order to pass, but the problem is the CEL that comes on. I asked AmericanMuscle about this and they said the highflow cats are a hit or miss in this situation. I have to get a ODB-II check soon so hopefully they'll take some dough to let me pass.
2011 Mustang GT black. Mods: JLT CAI, JBA Long Tube Headers Catted H-pipe, GT500 axle back, Bama 93 Race tune, Koni STRTs, Speeda Sports, GT500 mounts, BMR LCAs, relo brackets and panhard bar