Coolant Explained: What It Does and When to Top It Up
Coolant does more than its name suggests, it's actually a year-round system that manages engine temperature in both directions, not just preventing freezing in winter.
What coolant actually does
It circulates through the engine absorbing heat, then releases that heat through the radiator, keeping the engine within its optimal operating temperature regardless of season. In cold weather, it also prevents the water content from freezing and expanding, which could otherwise crack the engine block. It contains corrosion inhibitors too, protecting metal components inside the cooling system from rusting over time.
Checking the level safely
Only ever check or top up coolant when the engine is completely cold. The system is pressurised when hot, and opening it can cause serious burns from escaping steam and hot coolant. Check the level against the min and max markings on the expansion tank, a translucent plastic reservoir usually near the radiator.
Signs of a coolant problem
- The temperature gauge running higher than normal
- A sweet smell, often noticeable before any visible sign of a leak
- Coolant visibly pooling under the car after it's been parked
- The level dropping steadily over weeks with no visible leak, suggesting it's burning internally
Never mix coolant types
Different coolant formulations aren't always compatible, and mixing them can cause a chemical reaction that damages the cooling system rather than just diluting the mixture harmlessly. If you're topping up and unsure what's already in the system, using the exact type specified in the handbook is the safe default.
Check a car's MOT history for cooling system related advisories.
Check MOT history