There is not a single Action Replay code for a completed Pokédex. Instead you have to enter each individual Pokémon's code by pressing L+R in game and entering each Pokémon's code.
Action Replay has been around since the Commodore 64 and has continued up through the Nintendo 3DS and Wii. Action Replay allows cheat codes to be entered in games such as infinite lives, super-high jumps, warping to any level, infinite money and instant kills. Action Replay also allows players to download, import and export saved games from the Internet. It has received criticism from players who blame corrupt save files on the Action Replay's cheating mechanisms.