What Is Engine Coolant and When Should You Change It?

What does engine coolant do?

Engine coolant (also called antifreeze) circulates through your engine to absorb heat and carry it to the radiator, where it's dissipated. Without adequate coolant, your engine overheats — which can cause warped cylinder heads, blown head gaskets, and in severe cases, complete engine failure.

Coolant also contains corrosion inhibitors that protect the metal surfaces of your cooling system — the radiator, water pump, hoses, and heater core. As these inhibitors degrade, corrosion begins.

How do you check the coolant level?

Never open the radiator cap on a hot engine — the system is pressurised and you'll be scalded. Wait until the engine is completely cold, then check the coolant reservoir (a translucent plastic bottle near the radiator). The level should be between the MIN and MAX marks.

Also look at the colour. Fresh coolant is typically bright blue, green, or orange depending on the type. If it looks brown, rusty, or milky, that's a warning sign — get it checked.

When does coolant need changing?

Most manufacturers recommend a coolant flush every 2 years or 30,000 miles, though some long-life coolants extend this to 5 years. Check your handbook for the specific interval for your car.

Regardless of interval, change it immediately if:

• The coolant looks discoloured, rusty, or has visible debris in it.
• There's oil contamination (the coolant looks milky — this can indicate a head gasket issue).
• You've had any cooling system work done.
• The car has been running hot.

Can you just top up instead of flushing?

Topping up with the correct coolant is fine if the level is low and the coolant is otherwise in good condition. But topping up doesn't replace the corrosion inhibitors or remove old, degraded coolant. A full flush removes old coolant, clears any debris, and refills with fresh fluid — protecting the whole system.

Not sure about your coolant? WhatsApp Reliance Autos — we check coolant condition as part of every service.