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.
Bonus Rounds
To play the bonus rounds at any time, insert any game into your Sonic & Knuckles cartridge. When the "No Way" message appears, hold A + B + C and press Start. If you do this with the original Sonic game you can play the levels in order and will be given a password to save your progress.
Note: In case you're wondering, I've left out the passwords deliberately. There are over 32,000 bonus rounds in Sonic & Knuckles. I can't possibly list them all on the page, nor is there much point to doing so since the levels are nearly identical. Thank you.
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.
Debug Mode when Linked to Sonic 3
First go to Angel Island Zone and, while on a vine, press Left, Left, Left, Right, Right, Right, Up, Up, Up. If it has been entered correctly, you will here a chime. Then press Start then A. At the Title Screen, scroll down to Sound Test and press Start. At the Sound Test screen, go to Mushroom Hill Zone and press Start. While on a ratchet machine, enter the same code you entered in Angel Island. You should here a chime. Again, press Start then A. At the Tiltle Screen again scroll down to Sound Test. At the Sound Test Screen select a zone while holding A. Hold until the name dissapears. You can check if you're in Debug Mode by pressing B. Your score should turn into hexadecimal characters. The controls are these:
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.
Get a Super Emerald
Do the Level Select code, if you don't know it, here it is. Start the game as Sonic, and go to the first pull-up machine and enter this code at the left of the pull-up machine (must be hanging), LEFT,LEFT,LEFT,RIGHT,RIGHT,RIGHT,UP,UP,UP, and you should hear a ring sound. Pause the game and press A to reset to the title screen. To go to the Level Select screen, hold A and START. It has Sonic 3 Levels, but don't go to them, coz it will take you to Mushroom Hill zone. For the Super Emerald, go to Special Stage 1, and complete the Special Stage with a Perfect (all the rings in the Special Stage). When you complete the stage , a Super Emerald will come up for Sonic to collect. But when you get all Chaos Emeralds, you have to do one more Special Stage thats NOT on the game.
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
Start Mushroom Hill as Sonic. Run to the right until you come to the first Pull-Up machine. (It's just after the box with the power sneakers and the Madmole.) Hang from it and press left left left, right right right, up up up. You should hear a ring sound. Pause the game and press A to reset to the title screen. Now hold A and press start for level select.
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 Select when Linked to Sonic 3
First off, do the regular Sonic & Knuckles Level Select. Once that is done, hold UP + A, then put Sonic 3 in and reset the game. When you get to the Sonic 3 & Knuckles menu you'll find that Sound Test is there.
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.
Secret Stages and codes
Lock on Sonic 1 or a non-Sonic game. If it works it will say NO WAY. Hit A, B, and C at the same time. It will have another screen. Press Start to play the stage. When you beat the stage, a ten-digit code appears. I have yet to figure out what that does. Some games won't work this magic, however.