While laying, pause the game then enter the following codes. They will only work on Single Player Campaign, Easy Mode.
Infinite Ammo:
RT, RB, Y, X, Right, Down, Left, LB, LT, RT, RB, Y, X, Right, Down, Left, LB, LT, RT, LT, LB, RB, Y, Left, Down, X, RB + LB.

Infinite Health (heat will still go down):
Down, Down, Down, Up, Y, Up, Y, Up, Y, Up, Up, Up, Down, X, Down, X, Down, X, Left, Y, Right, X, Left, Y, Right, X, RB + LB.

500 Thermal Energy:
Up, Up, Down, Down, Left, Right, Left, Right, X, Y, RB + LB.