Enter the following codes at the Campaign / Bonus Features / Credits menu..

Code Effect

Down, Up, Left, Right, Right, Right, Up, Down Unlock All Mission
Left, Left, Up, Left, Right, Down, Right Infinite Health - Invincible
Right, Left, Right, Left, Right, Left, Right No vehicle's running on the street and no tank's/cop's attack
Right, Up, Up, Down, Right, Left, Left Unlock Cybertron missions
Down Right Left Up Down Down Left Unlock G1 Optimus Prime
Left, Up, Down, Down, Left, Up, Right Unlock Generation 1 Jazz Repaint
Right. Down, Left, Left, Down, Up, Up Unlock Generation 1 Starscream Repaint
Down, Down, Up, Up, Right, Right, Right Unlock Robovision Optimus Prim
Down, left, left, Down, Right, Right, Up Unlocks G1 Megatron

Unlockables

G1 Jazz Repaint Clear all sub- missions in Autobot and Decepticon story modes
G1 Megatron Collect all of the Decepticon Icons on all of the maps.
G1 Optimus Prime Collect all of the Autobot Icons on all of the maps.
G1 Starscream Repaint Beat the Deceptacon's story mode
Robovision Optimus Prime Beat the Autobot's story mode