Tony Hawk's Underground Codes -

 

In-game reset:
Hold + + + during game play.

Unlocking cheats:
Select "Options" at the main menu and scroll down to find the "Cheats" option. There are fifteen cheats, but they are all off. To unlock the cheats, play as a pro in arcade mode. Finish all the goals for every level. Each time you do this, another cheat will unlock. Then, go back to the cheats menu to turn them on.