Welcome to the FX codes
 
Blue codes are old, they can be found everywhere or in a few places.
 
Red codes I've found in mags and sites but I can't get them to work. (they are probably fake)
 
Green codes- I don't have this game so I can't test it.
 
Orange codes are discovered by me, you might find them elsewhere but they were here first.
GENESIS
Sonic &
Knuckles
Stage Select & Sound Test - While hanging on a elevator lift press Left, Left, Left, Right, Right, Right, Up, Up, Up(ring) Start, A
 
(Lock on Sonic 2)
 
Stage Select, Sound Test & Extra Options During Game - At title press Up, Up, Up, Down, Down, Down, Left, Right, Left, Right(ring)            Hold A, Start for Stage Select & Sound Test
When paused    A=Reset    Hold B=Slo-Mo    C=Frame Skip
 
Debug - At Sound Test play songs 1,9,9,4,1,0,1,8(ring)hold A when selecting a level
B=Enter/Exit Debug    A=Change Item    C=Place Item
 
Green and Black Knuckles - In Emerald Hill enter debug and turn into the item that looks invisible, keep hitting Right, C and place at least 60 waterfalls, hold Left for a 5 seconds, exit debug then hold right to run through the waterfalls. (if it doesn't work add more waterfalls)
 
Easy Act End - In Debug find the thing that looks like jumble of metal, place it and jump on top, animals come out and you passed the act
 
(Lock on Sonic 3)
 
Put the sign through the ground - In Carnival Night Act 1 hold Down after beating the boss (this will not work with the Mega hit series)
 
Stage Select & Sound Test - On a swinging vine press Left, Left, Left, Right, Right, Right, Up, Up, Up(ring) Start, A
Press C to change the yellow numbers     0=Sonic & Tails     1=Sonic     2=Tails     3=Knuckles
When paused    A=Go to stage select    Hold B=Slo-Mo C=Frame Skip
 
Debug - Use Stage Select & Sound Test to go to Mushroom Hill Zone then while hanging on a elevator lift press Left, Left, Left, Right, Right, Right, Up, Up, Up(ring) Start, A,  Hold A when selecting a stage
C=Reverse Gravity     B=Enter/Exit Debug
(in debug)A=Change Item     C=Place Item
 
Erase all files - Put the Sonic & Knuckles cart in the genesis now take the Sonic 3 cart, turn it backwards and plug it into S&K, flip the power on then off and all save data will be erased
(I must have been a complete idiot when I tried this, If you are dumb enough to try this, don't come crying back to me if you fry anything)
 
Knuckles Only
 
Marble Garden 2: Fight Sonic/Tails Enemy - At the face you can't hit, you have to go back to the nearest top spinner (super knuckles + a top spinner will break the wall down) after that knuckle glide off the spikes to hit the target
 
Carnival Night 2: Sonic/Tails Enemy - Climb last vacuum tube on the left side{hard but possible} you must take the Sonic/Tails stage (the easiest way is glide off of the top of the first barbar pole and find the vacuum tube)[hit the brakes when going through fan tunnels, sometimes you go through the ramp and you want the hi-way]
 
Use These With Debug
 
Sonic & Tails
 
Angel Island 2: Fight Two Robotniks - Follow the bottom of the screen (Knuckles' stage) go past the fight scene with Knuckles and the chase scene starts (sort of) after this you will be fighting two Robotniks (sometimes underwater)
 
Marble Garden 2: Fight Knuckles' Enemy - Use Debug to follow the Knuckles stage
 
Launch Base 1: Fight Knuckles' Enemy - Make the right half of the numbers say 07D6 and go right until you see a star post
                         0757
 
Launch Base 2: Knuckles' Stage - In Debug go down (Enter then Exit Debug for air)
 
Launch Base 2: Crazy Ending - Pass barricades with debug to fight Knuckles' enemy (game freezes but Start, A still works)
 
Lava Reef 2: Boss Skip - Take Knuckles' path
 
Tails only
 
Hidden Palace: Cool Master Emerald - After beating Knuckles, enter debug, go down for a couple seconds, then right until you hear two bounces, hold up a few seconds then B twice to move the camera up. You should see Robotnik taking a Master Emerald flashing all different colors
 
Screwed up ending - Enter Animation Test while falling and exit on the plane
 
Knuckles Only
 
Angel Island 2: Sonic/Tails Stage - At beginning of Act 2 move Debug cursor up (path leads to dead end)
 
Hydrocity 2: Big Time Glitch - Where you punch out the wall, use debug to drop below the bridge
 
Marble Garden 2: Fight Sonic/Tails Enemy - Use debug to drop below the bridge
 
Carnival Night 1: Sonic/Tails Stage - At beginning of Act 2 move Debug cursor up
 
Carnival Night 2: Sonic/Tails Stage - Where you punch out the wall, move Debug cursor up
 
Carnival Night 2: Sonic/Tails Enemy - Climb last vacuum tube with debug or climbing
 
Ice Cap 1: Weird Backgrounds - Go up Sonic's snowboarding path
 
Ice Cap 1: Sonic/Tails Stage - Pass blockers with debug
 
Launch Base 2: Sonic/Tails Stage - At beginning of Act 2 move Debug cursor up
 
Launch Base 2: Sonic/Tails Enemy - Where you punch out the big squares, move Debug cursor up
 
Launch Base 2: Dry stage: Use debug to pass the pipes without breaking them
 
Stage Select Secrets
(some of these require debug to work)
 
Knuckles Only
 
Lava Reef 3 - Sonic/Tails stage to fight robotnik
 
Sky Sanctuary 1 - Sonic/Tails stage "Knuckles Cloned" (leads to Death Egg 1)
 
Death Egg 1 or 2 - Sonic/Tails stage (to hit the act 1 boss, jump, enter debug, put cursor on the eye and exit)
 
Doomsday 1 - Screwed up Hyper Knuckles with a screwed up ending (game freezes)
 
Sonic & Tails
 
Sky Sanctuary 2 - Knuckles Stage (tail plane ending as tails)
 
Tails only
 
Doomsday 1 - Screwed up Hyper Tails (during last fight, enter then exit debug to go 2x speed)