HeartGold uses a 512KB save file format. Some older flashcarts or emulators default to smaller sizes, causing your progress to disappear after a restart.

A legitimate Pokemon HeartGold file will end in .nds , .zip , or .7z . Never run a .exe or .msi file.

You must use a "patched" version of the ROM or use an emulator (like DeSmuME or MelonDS) that automatically bypasses these checks.