None of them are working for me, why is this? am i doing something wrong? cuz im doing what it says...

What cheats are you trying to use ? Besides getting nitrous for the offline game, I don't remember any codes for Sega GT online.

cheats? what cheats?

I tried some of the cheats that were listed in the Sega Cheats forums and only the NOS worked.

Don't matter, the games was fun anyway.