Sonic The Hedgehog - Cheat mode:

At the Sonic The Hedgehog title screen, press Up, Down, Left, Right. Hold B for level select and sound test.

Sonic The Hedgehog - Debug mode:

At the Sonic The Hedgehog title screen, press Up, X, Down, X, Left, X, Right, Up, X, Down, X, Left, X, Right, X, Now hold B and press Start. Press A to enter debug mode.

Sonic The Hedgehog 2 - Level select:

Go to Sound Test option. Choose sounds 19, 65, 9, and 17. Hold X and press Start to return to the title screen, then hold B and press Start to access the level select menu.

Sonic The Hedgehog 2 - Super Sonic:

Enable the "Level select" code and go to Sound Test option. Choose sounds 4, 1, 2, 6. Start a stage, collect 50 rings, then jump to transform into Super Sonic.

Sonic The Hedgehog 2 - Debug mode:

Enable the "Level select" code and go to Sound Test option. Choose sounds 1, 9, 9, 2, 1, 1, 2, 4. Select any starting level, then hold B + Start until play begins. Screen coordinates will appear instead of a score and the number of objects displayed will appear instead of the time remaining. To edit the level, press the button configured as B on the Genesis. Sonic should become an object somewhere on the level. Press the button configured as C to "paste" that object in the level. Press the button configured as B again to resume game play.

Sonic and Knuckles - Level select:

Start a game with Sonic and run over to the Madmole near the Super Shoes power-up. Directly to the right of the Madmole is a pulley mechanism that takes you to the next level above by pushing Down on it. Instead of pushing down, push the Left, Left, Left, Right, Right, Right, Up, Up, Up. Now press Start to pause the game and press B. You will be sent back to the main screen. When you can select between Sonic and Knuckles, hold B and press Start for a level select.

Sonic The Hedgehog 2 And Knuckles - Level select:

At the Sonic The Hedgehog 2 title screen, press Up, Up, Up, Down, Down, Down, Left, Right, Left, Right, press and hold B, and then press Start. A stage select screen will appear.

Sonic Spinball - Level select:

At the Main Menu, select Options. Now at the Options screen, press A, Down, B, Down, X, Down, A, B, Up, A, X, Up, B, X, Up. You should hear a sound to confirm. Return to the main menu and then enter the following codes to unlock the corresponding level.

Level 2: Hold A and press Start
Level 3: Hold B and press Start
Level 4: Hold X and press Start

Sonic 3D Blast - Level Select:

When "Press Start" appears on the Sonic 3D Blast title screen, press B, A, Right, A, X, Up, Down, A, Start.

Unlock Flicky Bonus Game:

Insert a memory card with a save file for Sonic Adventure 2 Battle, and Flicky will be available at the Games menu.