Sonic The Hedgehog

Level select:
Press Up, Down, Left, Right at the title screen. A chime will confirm correct code entry. Hold A + Start to begin game play. In the Japanese version, press Up, Down(3), Left, Right at the title screen. Hold A + Start to begin game play.
Information in this section was contributed by Pat and Oliver Maanus.

All Chaos Emeralds:
Enable the "Level select" code. Select the "Bonus level" option on the level selection screen. Obtain the Chaos Emerald, reset the game, and repeat the process until all of them are in your possession. Allow the game to continue after obtaining the last Chaos Emerald to start in the Green Hill Zone.

Note: When you collect all of the Emeralds, you will not become Super Sonic, but you will be practically invincible. Try the following. When you have no rings, get attacked by something. Sonic will fall back (as if he had a shield on him), but he will not die. However, Sonic will die if he falls off of the edge of something, sinks underwater, etc.
Information in this section was contributed by Chaotix.

Debug mode:
Press Up, C, Down, C, Left, C, Right, C, Start, then hold A at the title screen. Continue to hold A until game play begins. Screen coordinates will appear instead of a score and the number of objects displayed will appear instead of the time remaining. Use one of the following controller actions to enable the corresponding debug code. Note: This code may only be enabled on the first version of the game.

Object transformation
Press B during game play to change Sonic into a game object. Press A to change into a new object. Press C to place an object on the screen. Press B again to revert back to Sonic. Note: The type of object that may be selectable will vary from level to level.

Invincibility
Sonic is, by default, invincible in debug mode. Note: Sonic can still drown, be crushed, fall and die, etc. He is only invincible to enemy attacks.
Information in this section was contributed by Fakey McFake.

Slow motion
Pause game play and hold B.

Restart
Pause game play and press A.

Frame advance
Pause game play and press C.

In the Japanese version, press press Up, C, Down, C, Down, C, Down, C, Left, C, Right at the title screen. A sound will confirm correct code entry. Pause game play, then press the buttons described above to activate slow motion, restart, and frame advance.

Crazy Sonic:
Hold A + B + C during demonstration mode for force Sonic to run into objects and act strangely. Note: This may also be done during the ending sequence after completing the Final Zone.

Bonus points:
Complete Green Hill Zone 1-1 or 1-2 in 29 seconds or less to receive 50,000 bonus points.

Unlimited lives:
Collect the extra life at the very bottom of Green Hill Zone - Act 3. Accumulate 100 rings to obtain another extra life. Collect a third extra life located at the top of the loop-the-loop. Intentionally lose a life and repeat the process.

Control ending sequence:
Enable the "Debug mode" code. Press B when Sonic is running with the animals at the end, then turn back into Sonic. You can nowcontrol yourself.
Information in this section was contributed by HOOHAZ.

Hint: Defeating the first Boss:
Hit him before he gets out his wrecking ball. While he swings the ball, jump (but do not hold it down) and hit him. Keep doing this until he is defeated.
Information in this section was contributed by Rjackrill.

Hint: Special Zone entrance:
You can almost always have the Special Zone entrance open (the giant ring next to the sign at the end of level Acts 1 and 2) by not losing your rings. If you do lose your rings, when you pass a checkpoint, intentionally die, then continue through the level without losing any rings. This works about 80% of the time.
Information in this section was contributed by SteinerofBTB.

Hint: Easy Emeralds:
Enable the "Debug mode" code. At the beginning of the level, create 50 rings and have Sonic collect them. Then, take Sonic to the end of the stage where you the giant ring. In the special stage, turn Sonic into an item and bring him to the end of the stage where the emerald is located. Turn him back and he will immediately get the Emerald. Repeat this until you have all six.
Information in this section was contributed by Bobby Gaines.

Hint: Get all the same color Emeralds:
Enable the "Debug mode" code. While in Special Zone, transform into something then go as far left and down as possible until Sonic stops shaking. Turn back into Sonic. You will fall to a place which is spinning. Look around to find every different Emerald. Note: If you fall off this place, you will fall back to your original location. To prevent this, transform and repeat the process. This works in every Special Zone.
Information in this section was contributed by Brianthebmoats.

Hint: Scrap Brain 3: Shortcut:
To pass Scrap Brain 3 in a very short amount of time, press the button at the beginning of the level, and jump over the bridge to go in the path below the bridge. Jump over all the pits. By the last pit, fall down, and go up the path on the right until you see the red jumper. The jumper brings you to the next level.
Information in this section was contributed by Eliyahu.

Hint: Green Hill Zone 3: Flicky reference:
On Green Hill Zone Act 3, just after Sonic steps on the cap to explode the rocket ship, animals pop out. One is a rabbit, and the other is a blue bird who appears to be Flicky, who is from another classic SEGA game, Flicky.
Information in this section was contributed by RyaMdwd.

Hint: Star Light Zone Act 1: Secret stash:
Enable the "Debug mode" code. If necessary, use the level selection menu to go to the level. Run to the first wall you find, which should have a bomb enemy. Press B button and go through the wall so that you are between two green walls. Either press B again or just fly down to find three ring boxes and an invincibility box. Note: If you use the invincibility and press B, all the sparkles will stay where they last appeared. If you press B again, you will continue the time from last time you pressed B. This lets you use invincibility whenever desired. Note: The time with invincibility will eventually run out.
Information in this section was contributed by Hedgehog1005.

Hint: Reset clock:
If you are running out of time, go to a checkpoint. Once you get the checkpoint, press Start and select "Quit". Then at the main menu, select "Restart". You will resume at the checkpoint with the time reset, allowing you to continue. Note: "Restart" is a new option that will appear when you select the "Quit" option during a level.
Information in this section was contributed by Chaos-chicken.

Glitch: Early unlimited lives:
Enable "Level select" and "Debug mode" codes. Put a lamppost just before the sign at the end of the level. Press B to transform back into Sonic and hit the lamppost. Get out until the lives read "1" at the bottom of the screen. Spin the sign to end the level. Wait until Sonic walks out of the screen. Enter debug mode and go under the sign to get out again. The "Game Over" message will appear while the credits are being shown. You now have unlimited lives.

Glitch: Secret underground hideout:
Enable the "Debug mode" code. Go to the end of Star Light Zone Act 1 without passing the sign. Press B and go under the floor at the end to find a secret underground hideout.
Information in this section was contributed by Hedgehog1005.

Glitch: Endless bounce:
Enable the "Debug mode" code. Go to Green Hill Act 2 and find the location with a spinning wheel, two or three falling blocks, and a floor of spikes. If needed, use "Debug mode" to find this area. Make all the blocks fall and land on the spikes, but away from the land at the end of the blocks. You should lose all of your coins and bounce forever until you reset. Debug mode is needed for default invincibility. For an alternate method, get all the Chaos Emeralds for invincibility for spikes enemies, etc.
Information in this section was contributed by Hedgehog1005.

Glitch: Cannot move:
Enable the "Debug mode" option. Collect fifty or more rings (or just transform in a ring and get fifty or more). Go to the end of the level and do not pass the ending sign. Press B button and look through your transformations. You should find the Big Ring (at the end with fifty or more rings) in your options for transformations. You can now place another Big Ring. If you place a Big Ring before you pass the sign and behind the sign then go inside the ring the game will glitch. You cannot move (including using Debug mode), see yourself, or hear yourself. Note: This was done in Green Hill Act 1;
Information in this section was contributed by Hedgehog1005.

Glitch: Left over graphics:
Enable the "Debug mode" and "Level select" codes. Select Special Stage at the level selection screen in Debug mode. Turn into an object and exit the compressed area (The place that Sonic is always in without Debug mode). When you think it is far enough, turn back into Sonic. You may seem to fall for awhile, but if you wait long enough you will find a "W" (possibly for the renamed Clock Work Zone, that was later renamed Scrap Brain Zone. They renamed the zone because they forgot the "W" in the games coding) labels that say 1 Zone; 2 Zone; and 3 Zone. There is also the Sonic !-Up as seen in the manual, but never seen in the Special Stage.