Begin the game and move Ecco left and right. Press Start to pause the game while Ecco is turning. You have to pause it when Ecco is directly facing you. When you do that, press Right, B, C, B, C, Down, C, Up. If you did it right, a cheat menu that has options such as stage select, message test, sound test, and more, will appear.
Invincibility
Input a valid password, then press Start, and wait for the screen that shows the name of the level you're currently in with your password. Next, hold A and Start. Keep hold of A and Start until Ecco appears on the screen. Now press Start to unpause and you will be invincible.
Level Passwords
Password - Effect
City of Forever - AABBRQLU
The Undercaves 2 - DQDIACCI
The Ice Zone - DWFFZBMV
Deep Water Zone - EILQOQLC
The Library - FDGXQQLC
The Vents - FKWLACCA
Open Ocean - FNCQWBMT
Pteranodon Pond - FQREUNLI
Ridge Water - HYAUGFLV
Dark Water - JNXPUNLA
The Tube - KUVEKMLK
Cold Water Zone - MCLFRQLW
The Lagoon - NDRBRIKR
Trilobite Circle - OBEMUNLX
Jurassic Beach - PLABUNLT
Hard Water Zone - QGDJRQLA
Origin Beach - QXKIUNLX
The Machine - SDDBKMLG
The Island Zone - UWXIOQLK
The Undercaves - UYNAINCC
Deep Water 2 - WSGAKNLC
The Marble Sea - XAKUQQLS
City of Forever 2 - ZBPIGPLD
Deep City - ZUVPQQLU