Blue Knuckles
Enable the Level select and Debug mode codes with Sonic 3 locked-on. As Sonic, go to Sky Sanctuary Zone Act 2(this is where Knuckles plays it). Turn into the Item box and place some on the screen. Beat Meatel Sonic with hyper on. You will see a Blue Knuckles in the ending.
Debug Mode when Linked to Sonic 2
Go to the level select screen and play tunes 1,9,9,4,1,0,1,8 (the date when the game was made.) Choose any level, then hold A and press Start to activate Debug mode. To use it, see the instructions in Sonic the Hedgehog.
Easy Emeralds when Linked to Sonic 2
Enter the LEVEL SELECT code shown below. Go to "special stage" and get an emerald. You will go to Emerald Hill Zone. Pause the game, and press A. Enter the level select again and return to the special stage. Repeat this process until you have all the emeralds.
Extra Points
When the end of act marker comes down, jump and hit it for 100 points. Repeat this to get more points.
Hyper Sonic, Hyper Knuckles or Super Tails
Connect your Sonic & Knuckles cartridge to Sonic 3. Activate the level select and debug. When you enter debug mode, create a monitor. It will have an "S" on it. Jump on it to get 50 rings and become Hyper Sonic, Hyper Knuckles, or Super Tails!
Knuckles with a tail
Do the Blue Knuckles code, but with Tails this time.
Level Select when Linked to Sonic 2
At the title screen, press Up, Up, Up, Down, Down, Down, Left, Right, Left, Right. You should hear a ring. Next, press A + Start to enter the Stage Select screen. This code will remain in effect until you turn off the Genesis.
Level Skip when Linked to Sonic the Hedgehog
Play any special stage. collect all the rings and you will be able to skip 10 levels.
Play at Night when Linked to Sonic 2 or Sonic 3
For Sonic 2: Activate level select (19, 65, 9, 17) and hold C and press Start.
For Sonic 3
Activate level select (Up, Up, Down, Down, Up, Up, Up, Up) and hold C and press Start.
Red Sonic
Do the Blue Knuckles code. When the credits play, turn into that same item. Place it on the screen and hit it. Press B twice. You should now be a red sonic. You will die, too.