There is no way to reverse cheats in Nintendogs. Once a cheat (Action Replay, flashcart, save editor) modifies your save, the changes are baked into the file.
Why Reversal Is Impossible
Direct Save Editing: Cheats rewrite the save sectors (money, trainer points, dog stats, items). No "undo log" exists.
No Cloud/Backup: The DS/3DS doesn't auto-backup saves. Unless you manually backed up the .sav file beforehand, the pre-cheat state is gone.
Anti-Cheat Flags: Some cheats set hidden "tampered" flags that persist even if you edit values back.
Your Only Options
Start a New Game: Fastest clean slate. You'll lose all progress, but the game is designed for replayability.
Accept the Modified Save: If it's just extra money/items, keep playing. The core gameplay (walking, contests, tricks) is unchanged.
Restore a Manual Backup: If you used a flashcart (R4, DSTWO) or homebrew (Checkpoint, JKSM) to dump the .sav before cheating, restore that file.
Prevention for Next Time
Dump your save weekly with Checkpoint (3DS) or GodMode9.
Play legitimately—Nintendogs is about the daily bond, not max stats.
Lesson learned: virtual puppies don't need infinite money to love you.
