At the 'Main' select the 'Cheat Codes' option and enter the following codes to unlock the corresponding effect.

Invincibility
Press R3, X, Square, L3, Square, Square

Lower Enemy Accuracy:
Press Square, L3, R3, L3, R3, R1

Melee Instant Kills:
Press R3, X, L1, O, R3, L1

No Special Cooldown Time:
Press R3, Square, R3, R3, Square, X

No Weapon Overheat:
Press L3, Square, X, L3, Triangle, L3

Unlock ALL Cairo Missions and Zones:
Press R3, Triangle, X, Triangle, L3, L1

Unlock ALL Deep Six Missions and Zones:
Press Square, R1, Triangle, O, X, L1

Unlock ALL East Coast Missions and Zones:
Press R3, L3, R1, X, O, Square

Unlock ALL Shanghai Missions and Zones:
Press Triangle, L3, R3, L1, Triangle, X

Unlock ALL West Coast Missions and Zones
Press L1, R1, R3, Triangle, R3, O

Unlock and activate ALL Upgrades:
Press L1, Triangle, L1, O, Square, Square

Unlimited Turbo:
Press O, L3, Square, X, Triangle

Always in Overdrive Mode:
Press L1, O, L1, X, Square, R3

Increased Enemy Accuracy:
Press Triangle, Triangle, O, X, Square, L1

Increased Enemy Damage:
Press L1, Triangle, X, Triangle, R3, R3

Increased Enemy Health:
Press O, Square, L1, O, R3, Triangle

Increased Weapon Damage in Robot Form:
Press Triangle, Triangle, R3, X, L1, Triangle

Increased Weapon Damage in Vehicle Form:
Press Triangle, O, R1, Square, R3, L3

Extra Energon (ex: 4x frrom defeated enemies):
Press Triangle, Square, O, R3, X, Triangle

G1 Colors Ironhide (Single Player only):
Press L1, R1, R1, X, O, Triangle

Low Gravity Mode (Everyone jumps higher and longer):
Press Square, Square, Triangle, L3, Triangle, L3

Play as Autobot Protectobot Scout MP in Autobot based Single Player (only when mission begins, not in character select) Does not work in Deep 6:
Press R3, L1, L1, Triangle, Square, X

Plays as Decepticon Seeker Warrior MP in Decepticon Based Single Player (only when mission begins, not in character select):
Press Square, Triangle, Square, L1, X, L1

Special Kills Only Mode (Cannot kill enemies except with special kills):
Press O, O, R1, O, X, L3

At the 'Main' menu select 'Cheat Codes' and enter the follow codes to unlock the corresponding playable character.

Unlock G1 Starscream:
Press O, X, O, R1, Triangle, R1.

Unlock Gold Megatron:
Press Down, Up, Right, RIght, Left, Up.

Unlock Gold Optimus Prime:
Press Up, Down, Left, Left, Right, Down.