I found a car repair cheat code,actually it is the same as the max health cheat code just used differently,here is the tutorial,you need to be standing next to your car but not in the car,then start tiping the max health cheat code but when you get to the triangle button,wait util he gets in the car
You don't need to do that, you can do it from the pause menu (also lets you repair planes in midair).
The first button of the health code will exit pause, so what you have to do is pause and then access the map (X on PS3). This way, when you hit the first button, it will back out of the map instead of exiting pause, and obviously the "enter/exit vehicle" button will do nothing since it's paused.
As soon as you enter the code, unpause the game, and the cheat will be applied. Don't hit any extra buttons afterward, just unpause.