The game expects a value between 0 (No Heat) and 5 (Maximum Heat).
To keep your save file stable while using editors, stick to these rules:
Most save editors allow you to manually input values for car heat. However, NFS Carbon uses a specific floating-point system for these numbers.
Never set car heat above 5.0; the game engine cannot render Heat Level 6+ without specific mods.
Are you trying to fix a in your career garage, or are you looking to unlock the hidden Heat Level 6?