To repair a vehicle, you must use 3 tools in a specific sequence, the sequence is:

Wrench > Screwdriver > Hammer > Wrench (again)

Each tool works on a specific health chunk, the table below shows you when to use which tool:

Damage range: 250-450 450-650 650-800 800-Max
Indicator colour: Red Orange Yellow Green
Tool to use: Wrench Screwdriver Hammer Wrench

While a vehicle's health is below 450 (Wrench range or Red) the engine will cut out and turn on randomly, ultimately slowing down the vehicle.

If a vehicle's health drops below 300, the engine will cut out completely and never come back on until the vehicle is repaired.