What is antifreeze? All about the coolant in your engine

Antifreeze, also known as coolant, plays an essential role in keeping your car’s engine working in both hot and cold conditions. Failing to keep your antifreeze topped up can cause significant damage to your car’s engine, potentially enough to make the car a write-off. Read on as we explain what antifreeze is and how to top it up.

As its name suggests, antifreeze stops the water inside your car’s engine from freezing. If the water were to freeze, it would expand and cause significant engine damage. In warmer weather, antifreeze helps to stop the engine from overheating and lubricates the pumps in the cooling system.

Engineers will often use the term ‘coolant’ to describe the fluid that consists of antifreeze and water. It’s pumped around the engine by the water pump and its job, particularly in hot weather, is to remove excess heat, thus preventing overheating.

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Coolant is used in liquid-cooled engines, so is found in every new petrol, hybrid or diesel-powered car you can buy today. Some air-cooled classics such as the Citroen 2CV, original Volkswagen Beetle and pre-1998 Porsche 911 don’t need any coolant. Some electric vehicles don’t need it either, although some of the more sophisticated models (such as a Tesla Model S and Audi e-tron) do have coolant fluid to help reduce battery temperature while charging.

How does antifreeze work?

Antifreeze is a liquid which has a much lower freezing point than water and is added to the car’s cooling system. The mixture is pumped around the engine by the water pump. As it passes through hotter parts of the engine, it absorbs that heat and the coolant is transferred to the radiator. The temperature of the coolant in the radiator is then reduced by the cold air that rushes through the radiator as the car drives along. A fan will cut in if the car is stuck in traffic and the temperature rises. After passing through the radiator, the coolant – at a lower temperature – is fed back through the engine where the process starts again.

If the weather is too cold, there’s a risk that the coolant could freeze, preventing it from flowing around the engine, causing blockages and also expansion damage. To combat this, antifreeze is added to the coolant. This is an alcohol-based substance that, when added to water, reduces its freezing point as well as increasing its boiling point. If your coolant contains the right concentration of antifreeze, your engine will be protected against almost any weather the UK can throw at it.

Types of antifreeze

Two types of antifreeze are widely available, but not all makes and models of car are compatible with both. You should always use the specific type recommended by the manufacturer of your car. This should be specified in the handbook, or it may be marked on the coolant tank in your car. You can also ask at your local dealership or contact the manufacturer’s customer services helpline.

Cars built from 1998 onwards typically need organic acid technology (OAT) antifreeze, which better protects aluminium components from corrosion. Cars built before 1998 are usually more suited to non-OAT-based antifreeze, which better protects copper and brass components. This is sometimes referred to as glycol antifreeze.

Things are made more complicated by the fact that different colours of antifreeze are also available, intended to work in specific cooling systems. Again, once you’ve identified the right type of antifreeze for your car, you should always use the same colour. Because the different colours of antifreeze correspond to different formulations, they’re not always compatible with each other.

Sometimes two non-compatible types of antifreeze won’t mix together correctly. When this happens, you can find elements of their makeup combining in solid lumps or gels, which can even block the cooling system if the problem isn’t spotted and resolved early.

If you’re in any doubt, it’s always wise to consult the owner's manual for your car or contact your dealer. Alternatively, most car spares shops should be able to sell you the correct antifreeze for your car. To do this, they’ll need the exact details of your car, including its make, model, engine size, fuel type and year of manufacture.

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You can buy antifreeze ready mixed to be added straight to your car, or as a concentrate that you can mix with water before adding it to your engine. The bottle will have instructions and mixture ratios printed on it – e.g. one part coolant to four parts water is the same as a 20% ratio of coolant to water.

To check your antifreeze, first make sure you have located the right tank under your bonnet – modern cars have several that look similar which can be confusing. The liquid inside should be a bright colour. If your antifreeze has lumps in it or is a dull brown colour, it’s likely to need changing. However, even crystal-clear coolant can lose its effectiveness against corrosion over time.

You can buy an antifreeze tester from most good car accessory shops. The cheapest type is a hydrometer (sometimes called a float tester) and these start at less than five pounds. A refractometer is more expensive at up to £50, but provides greater accuracy.

Although the two types work differently, both will tell you how cold your coolant can go before it’ll freeze. Most antifreeze provides an operating range from over 100 degrees Celsius down to below minus 30 degrees Celsius. The normal operating temperature of most car engines is around 90 degrees Celsius.

What happens to an engine when it runs out of coolant?

Because coolant keeps your engine at the right temperature, running out means your engine could quickly overheat. A dashboard warning light - and possibly steam coming out of the bonnet - will alert you to overheating. The warning is in place to minimise damage to the engine.

If you ignore the warning and continue to drive without coolant, it can lead to problems with parts of the engine, including the heads, head gaskets and the block itself. Major problems with these are likely to lead to huge repair bills or the car being written off.

Can I use water instead?

No. Never use water from a tap, hose or bottle in your cooling system. As well as offering protection against boiling and freezing conditions, proper coolant has a much reduced risk of corroding the metal parts of your engine that it contacts. The water pump, radiator and heating system, as well as the engine itself, can all corrode in prolonged contact with normal tap water.

If you’ve bought concentrated antifreeze, you’ll need to mix it with water following the instructions supplied. If you live in a hard water area (you’ll have noticed scale around your taps and in your kettle if you do), you should mix the antifreeze with distilled water, which has fewer impurities than tap water.

How do I top up car coolant/antifreeze?

Once you’ve tested your antifreeze and determined that it needs topping up, rather than replacing, you can follow our tips here. If your coolant level is regularly dropping between services, then you may have a leak and it’s worth investigating.

You should also top up fluids like antifreeze before storing the car off the road for longer periods of time. Check out our guide on how to store a car for more info.

Changing antifreeze

Your local garage will be able to change your antifreeze for between £50 and £100 with a flush of your car’s coolant system, although a main dealer could charge more. We recommend you call in the experts, as collecting and disposing of antifreeze (see below) can be messy. It’s also a good idea to have the cooling system flushed through to ensure it works most effectively.

Antifreeze disposal

You should never pour old antifreeze down a drain or pour it on the ground, because it’s highly toxic. Most local waste recycling centres will take antifreeze. Antifreeze poisoning is extremely hazardous to all animals, even in tiny amounts.

Read our in-depth guide to main dealer servicing, or visit our used car buying checklist.