The following are known Action Replay Codes for Scooby-Doo! Mystery Mayhem on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
3ee5f676
ae22accf
4632b690
44e9fdc6

Infinite Lives
9b64d35c
8a4f3114

Infinite Item Bar
eb5885c6
cdf0c916