Engine coolant — also called antifreeze — is one of the most important fluids in your car, responsible for keeping the engine at the correct operating temperature in both summer heat and winter cold. Despite its importance, coolant is one of the most neglected maintenance items, and many drivers do not know when it needs changing, what type to use or what happens if the system runs low or fails. In 2026, here is the complete guide to engine coolant.

What Does Coolant Do?

Engine coolant circulates through the engine block and cylinder head, absorbing heat from the combustion process and carrying it to the radiator where it is dissipated into the air. Without coolant, engines would overheat within minutes. The coolant also contains additives that prevent corrosion inside the cooling system and lubricate the water pump.

The antifreeze component prevents the coolant from freezing in cold weather. A 50/50 mixture of coolant and water freezes at approximately minus 37 degrees Celsius, providing protection in even the coldest UK winter conditions.

Coolant Types and Colours

Coolant comes in different types, identified by colour. The colour is not just cosmetic — different coolant types use different chemical compositions and are not always compatible:

  • Organic Acid Technology (OAT) — typically orange, red or pink: Long-life coolant used in most modern cars. Can last 5-10 years or 150,000+ miles. Not always compatible with older coolant types.
  • Hybrid Organic Acid Technology (HOAT) — typically yellow or green: Combines traditional and organic acid technologies. Used by several manufacturers including Ford and Volvo.
  • Traditional IAT (Inorganic Acid Technology) — typically green: Older coolant type, requiring change every 2-3 years. Still used in some older vehicles and some commercial applications.
  • Phantom colours: Some manufacturers use specific colours for their own coolant specifications — always check the owner's manual.

Coolant Change Intervals

Coolant change intervals depend on the type of coolant and the manufacturer's specification:

  • Modern OAT coolants: Up to 10 years or 150,000 miles in most cases
  • HOAT coolants: 5-7 years depending on manufacturer
  • Traditional IAT coolants: Every 2-3 years

Always follow the manufacturer's service schedule — this is specified in the owner's manual and service log book.

How to Flush the Cooling System

  1. Allow the engine to cool completely — never open the cooling system when hot
  2. Place a drain tray under the radiator drain plug or the bottom radiator hose
  3. Open the drain and allow the coolant to drain completely
  4. Close the drain and refill with distilled or deionised water
  5. Run the engine to operating temperature and allow to cool
  6. Drain again and refill with the correct coolant mixture
  7. Bleed the cooling system by running the heater on full heat with the engine running

Mixing Coolant Types

Mixing incompatible coolant types can cause chemical reactions that form sludgy deposits, accelerate corrosion and block coolant passages. If you are unsure of what coolant type is in your car:

  • Check the owner's manual for the specified coolant type
  • Look at the colour of the coolant in the expansion tank — but note colours are not always reliable
  • When in doubt, flush the system before refilling with the correct type

What Happens If You Run Out of Coolant?

Running even a short distance without sufficient coolant will cause the engine to overheat. The consequences range from minor — a small loss of coolant from a slow leak — to catastrophic — a blown head gasket or cracked cylinder head. Modern engines with aluminium heads and blocks are particularly susceptible to heat damage, and overheating can cause thousands of pounds of damage in minutes.

If the temperature gauge shows the engine is overheating, pull over immediately, switch off the engine and allow it to cool before adding coolant.