Instructions
Enter all cheats codes at the very beginning of the game while the main menu is up. Do this before you even begin playing the game; if you've gone past the main menu into the game proper, then you've already gone too far.
Enter all desired codes fairly quickly. It's not always enough to have entered the code at the right time or even in the correct sequence. Sometimes pressing the exact button combination too slowly will be enough for the game not to honor your cheat code. For example, for the Xbox version of the game, the code to unlock the Acura Integra is "BLACK," "BLACK," "R," "BLACK," "WHITE," "L," "DOWN" and "UP." Enter it fast -- but not too hastily -- for best results.
Verify that the cheat codes you use match those available for the specific gaming console on which you're playing "Need For Speed: Underground." For instance, the Nintendo Gamecube version of "Underground" features altogether different cheat codes than the Xbox or PlayStation 2 versions of the game, so make sure you've got the right codes for the right gaming system.