Most legacy GameShark and Action Replay codes for Pokémon FireRed do not just change the experience variables; they continuously overwrite the game's RAM. This brute-force approach leads to several notorious issues:
Because Pokémon FireRed has different regional releases and revisions (v1.0 vs. v1.1), you must match the code to your specific ROM version. pokemon fire red exp multiplier x2 fixed
While true "x2" multipliers are rare and often buggy, the following code functions as a high-speed multiplier. Most legacy GameShark and Action Replay codes for