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How to check and change your car’s coolant

Where coolant goes

Rob Keenan is the interim digital editor of
He runs a Mk2 Ford Focus ST and an aging Mercedes SLK55
Find him on Twitter @zorba_t_greek

Your car’s coolant is vital for the engine's operation. It not only keeps it at the right temperature but it helps to prevent corrosion, with rust inhibitors that extend the engine's life. So even if you live in a hot country, where there's no chance of freezing weather, you should never fill the coolant system with just water.

A typical coolant system comprises a series of pipes that run between the engine block, the radiator and an expansion tank. Other important elements are radiator fan(s) and the thermostat, which come into play once the coolant is up to operating temperature. Learn how your car's radiator works.

The fluid's level in the expansion tank will vary depending on whether the engine (and therefore the coolant) is hot or cold, which is why you'll usually see a 'Max' and 'Min' stamped on the side of the opaque tank in the engine bay (don't get this muddled up with the screenwash tank).

In an ideal world you shouldn't need to top up the coolant, but hoses and seals eventually degrade and leaks occur. Head here to find out why coolant can cause issues with your car's heater.

When should I change my car’s coolant?

If you're changing the coolant as part of a standard service procedure, you'll get different answers depending on who made your car and which coolant you use. If you're running a classic car and it uses Inorganic Acid Technology coolant, which is often blue, it'll need to be renewed every couple of years. Don't be tempted to switch to Organic coolant (see below), because it could damage your car's seals.

Most manufacturers of modern cars recommend 10-year intervals. In the case of the author's SLK55, Mercedes-AMG recommends changing the coolant (known as Organic Acid Technology) every 15 years. It's usually – but not always – pink. Every vehicle is different, so consult your Haynes manual or get in touch with your manufacturer's customer services department for advice. 

Of course, if you're having to change a component that's part of the cooling system, such as the radiator, thermostat or water pump, you're going to lose at least some coolant. Once the new part is fitted, you could top up the coolant, making sure you're using the same-grade, or completely drain the system down and refill it.

Wondering what type of coolant to use in your car and how to change it? Haynes shows you how to replace your coolant here.

Watch this video to see how to change your coolant