Coolant plays a vital role in maintaining the optimal performance and longevity of your car’s engine.

Without it, your vehicle would overheat and stop running altogether.

Read on and find out what coolant is, what it does and how to check and top up fluid levels.


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What Is Coolant?

Coolant is a term used to describe a fluid that is antifreeze diluted with water.

It works alongside the liquid cooling system to keep your engine from overheating or freezing.

Car coolant is created by mixing antifreeze (ethylene glycol or propylene glycol) with additive packages (e.g. corrosion inhibitors) and deionised water.

Ethylene glycol is highly toxic, and manufacturers have started using a less toxic version in recent years.

These are propylene glycol-based coolants.


What Does Coolant Do?

Coolant transfers heat away from and adds antifreeze protection to your engine.

When working well, coolant protects the metal, rubber and plastic parts in the engine and coolant system.

It prevents the engine from overheating or freezing when switched off, as it has a high boiling point and low freezing point.

It also acts as a lubricant, preventing damage to the water pump, head gasket, cylinder and piston timing.

Coolant does this as part of the liquid cooling system, which includes the:

  • Water pump
  • Radiator
  • Radiator hoses
  • Fan
  • Thermostat

The water pump circulates coolant around the engine.

The radiator conducts heat away from the coolant.

The radiator hoses connect the different parts of the cooling system.

The fan pulls air through the radiator if the car is not moving fast enough to move the air itself.

The thermostat controls the temperature of the coolant.


How Do You Check and Top Up Engine Coolant?

You should check your engine coolant every 2 to 3 weeks so that you can spot potential problems early on.

Most modern cars have an engine coolant dashboard warning light that can light up to alert you to coolant problems.

You should have your vehicle inspected by a professional mechanic if this light comes on.

To check or top up your engine coolant manually, you must first check the engine is cool before opening the coolant cap.

Wait at least 2 hours after your engine has switched off – or even longer if your engine has overheated.

Refer to your vehicle handbook to find the expansion tank and coolant filler cap.

Please note that adding coolant to the wrong reservoir could cause serious damage.

Next, check the coolant levels – these should be between the minimum and maximum marks on the side of the expansion tank.

Low coolant levels can cause damage to the car heater, in which case you might notice cool air coming from the dashboard vents even when the dial is set to ‘hot’.

Make sure that you check any hoses and look out for coolant leaks and any wet or white staining that could signal that something is wrong.

If your coolant needs topping up, only do this when the engine is cold – you could be scalded if not!

Use the right coolant for your vehicle – different types should not be mixed.

You should have your car looked at by a professional garage if you notice a drop in coolant levels.


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Can I Refill Coolant Myself?

If you are planning on refilling engine coolant yourself, you must make sure that the engine has completely cooled before opening the coolant filler cap.

Most modern cars have a sealed cooling system that shouldn’t need topping up unless there is a leak.


Which Coolant Does My Car Need?

There are different engine coolants available and you must buy the right one for your vehicle.

You can refer to the vehicle handbook to find out which kind your car needs.

Each manufacturer uses a different type, and there are even specialised versions of coolant on top of that.

These are for high-mileage vehicles or for use in a specific manufacturer's cars.

You can identify the different types by their colour:

  • Bright Green = Inorganic Acid Technology Coolant (IAT)
  • Orange = Organic Acid Technology (OAT)
  • Yellow = Hybrid Organic Acid Technology (HOAT)
  • Turquoise = Phosphate-free HOAT
  • Purple = Silicated HOAT (Si-OAT)
  • Blue or Pink = Phosphated HOAT (P-HOAT)

IAT was the standard coolant for older vehicles. It needs to be changed more often because it tends to lose its cooling properties quicker. This type can be yellow as well as bright green.

OAT provides high-temperature protection for aluminium and is found in many modern cars. This can also come in dark green, pink and blue.

HOAT is a combination of the other forms of coolant and has an extended lifespan. It can also come in orange and is popular in modern vehicles.

Si-OAT is made from a mix of silicates and organic acids.

P-HOAT is made from a mix of phosphates and organic acids.


You must use the right type of coolant for your car, or you could unintentionally cause corrosion and damage.

Most of the time, radiator failure is actually caused by a failing cooling system.

This usually happens if you used the wrong type of coolant 6 to 12 months ago.

It takes a while for the damage to become severe enough to become noticeable and many drivers forget their mistake and blame the radiator instead.


Is My Coolant Level Low?

To check if your car’s engine coolant level is low, you will need to locate your car’s coolant tank.

This is usually marked with a cap that says either ‘engine coolant’ or ‘never open when hot’. It could also display a symbol with an exclamation mark on it, or a combination of all three options.

Look at the tank and check if the coolant level is low – it will be below the ‘min’ line if so.

You should top up the coolant to between the ‘min’ and ‘max’ lines, when the engine is cool.


What Happens When You Run Out?

If your car runs out of coolant, then you could risk doing serious damage to the engine.

Your vehicle may suffer from a blown head gasket and the engine could overheat.

You should stop driving so that you don’t end up with an expensive repair.

To avoid these problems, be sure to book a professional coolant change when needed.


Do I Have a Leak?

If your engine coolant level is low or you can smell a sweet scent, then you could have a coolant leak.

There may be a puddle underneath the car if this is the case. To find out, check for a puddle after the car has been parked for 15 minutes.

The puddle will be the same colour as the coolant in your engine bay and smell sweet.

Open the bonnet to try and find the source of the leak.

Smell to find where the scent is coming from – if it is strong inside the vehicle, then the leak could be in the heater.

Whether you can pinpoint the leak or not, you should book an appointment with an expert as soon as possible to confirm and fix the source of the problem.


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Can I Drive with the Coolant Warning Light On?

If you notice an amber or red engine coolant symbol – which looks like a thermometer or a liquid tank – then you should check your coolant and top it up if needed.

Alternatively, you can have your vehicle checked by a mechanic.

An amber warning light means that the coolant level is low.

A red warning light means your car is overheating or that the coolant level is below the minimum mark.

Either way, you should top up before you keep driving – otherwise you could do serious damage to your car.

We recommend taking your car to a mechanic if your coolant level is low, as there may be a leak. 


How Often Should You Change Engine Coolant?

For most vehicles, engine coolant should be changed every 30,000 miles or 50,000 km.

This can vary depending on your vehicle, as some cars need the coolant changed much more infrequently. 

Check the owner's manual to determine the recommended interval for your car. 


How Often Should You Top Up Coolant?

You should top up your vehicle's coolant level whenever it drops below the guide marks.

If your engine coolant is going down quickly, there could be a cracked hose or hole in the radiator which a mechanic needs to inspect.


Can You Put Water in Coolant?

You should not top up your engine coolant with water, unless the coolant level is lower than it should be, and it is an emergency.

You can put water in the coolant tank if it already has coolant in it, but you should not put water in if it is empty.

Water cannot be used as engine coolant, as the water would evaporate and cause the engine to overheat.

In an emergency, you can top up your coolant with a little bit of water if it will help you get to the nearest garage.

You should only top up when the engine is cold.

If the antifreeze dilution level is too weak over the winter, the water in the cooling system could freeze.

Topping up coolant levels with water in the summer is even more risky, as your car will heat up quicker.

If this happens to you, we suggest that you have a mechanic come to you to check the car.

Alternatively, you can top up with the right coolant if you have it with you.


Why Is My Coolant Brown?

Coolant should be transparent or a bright colour, and you must always use the right type for your car as outlined in your vehicle handbook.

Brown coolant can be the result of a radiator leak, a clogged cooling system, low fluid levels, poor oil quality or metal particles being present in the oil.

If the coolant is dark brown or black, then this can suggest that engine oil is present in the coolant, pointing towards a larger head gasket issue that needs to be addressed.

You may also need to book a coolant change if the coolant is opaque or has bits floating in it.

Don’t ignore this problem – your engine could overheat and cost you thousands of pounds to repair!


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