Coolant is a vital part of any mechanical system and your car's engine is no different. Without it, we would struggle to drive anywhere because our engines would be constantly overheating. But what is coolant and what does it actually do?

By the end of this article, you will understand:

  • What coolant is and what it does
  • What it should look like
  • How to identify if your car is leaking coolant
  • What type your car needs and how to identify it
  • What the warning light looks like
  • How to solve any problems with your coolant



Chemically inert - A substance that doesn't gain or lose electrons during chemical reactions. This means that it doesn't react when exposed to extreme heat or form compounds with other elements.

Viscosity - How thick a liquid is. Viscosity is usually measured by the speed at which a liquid moves from one place to another.

Ethylene glycol - A sweet-tasting, odourless and colourless liquid used as an organic compound in antifreeze and polyester fibres. It is made of 6 parts Hydrogen, 2 parts Oxygen and 2 parts Carbon (C2H6O2).

Propylene glycol - A nearly odourless, colourless synthetic liquid that absorbs water and is used as an additive in certain medicines, cosmetics or food products, as well as antifreeze. It is non-toxic and recognised as generally safe by the Food and Drug Administration (FDA). It is made of 3 parts Carbon, 8 parts Hydrogen and 2 parts Oxygen (C3H8O2).



Coolant works alongside the liquid cooling system to keep your engine from overheating or freezing. It should be transparent and a bright colour of some kind. You must always use the right type for your car, which you can find in your vehicle handbook. If you check your coolant and the level is very low, or there is a sweet-smelling puddle under the car, you probably have a coolant leak that needs fixing.

Similarly, if your coolant looks brown, opaque or has bits floating in it, you need to book a coolant change at a garage. Ignoring a problem can cause your engine to overheat and the coolant warning light to come on. If you ignore this as well, you can cause expensive engine damage which can cost thousands of pounds to repair.


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What is Coolant?

Coolant is used to control the temperature of a system. It is usually a liquid or a gas and must have several key properties. These are:

  • Chemically inert
  • Cheap
  • A high boiling point
  • Low viscosity
  • Non-toxic.

Car coolant is created by mixing ethylene glycol with additive packages and water. However, ethylene glycol is highly toxic so, in recent years manufacturers have started using a much less toxic version. These are propylene glycol-based coolants.

As coolant is antifreeze diluted with water, you can add diluted antifreeze to the tank to offer your engine more protection from the cold.


What Does Coolant Do?

Its main jobs are to transfer heat and add antifreeze protection to your engine. It prevents your car from freezing and boiling and keeps your oil in liquid form. When working properly, it protects metal, rubber and plastic parts in the engine and coolant system. 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. This also includes the:

  • Water pump. This circulates the coolant around the engine.
  • Radiator. This conducts heat away from the coolant.
  • Radiator hoses. These connect the different parts of the cooling system.
  • Fan. This pulls air through the radiator if the car is not moving fast enough to move the air itself.
  • Thermostat. This controls the temperature of the coolant.

If you try to drive your car without this system, your engine is likely to overheat. This can cause expensive damage if you don't stop driving immediately. That's why it's so important to look after your coolant and ensure it is always in good condition.


What Should Coolant Look Like?

While coolant comes in a variety of colours, it should always be transparent. Most importantly, it should never be brown.

If you notice that your coolant is opaque, dark brown in colour and/or has lumps floating in it, you must flush and refill the system immediately. If you don't know how to do this, seek professional help and book a coolant change at a garage near you.



What Coolant Do I Need?

Coolant comes in several different forms. 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). This was the standard coolant for older vehicles. Its biggest downside is that it needs to be changed more often because it tends to lose its cooling properties quicker. Can also come in yellow.

Orange = Organic Acid Technology (OAT). This provides high-temperature protection for aluminium and is found in many modern cars. Can also come in dark green, pink and blue.

Yellow = Hybrid Organic Acid Technology (HOAT). A combination of the other forms of coolant, HOAT has an extended lifespan and is also popular in modern vehicles. Can also come in orange.

Turquoise = Phosphate- and NAP-free HOAT.

Purple = Si-OAT (Silicated HOAT). Made from a mix of silicates and organic acids.

Blue or Pink = P-HOAT (Phosphated HOAT). Made from a mix of phosphates and organic acids.


You must use the right type for your car, or you can 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 6-12 months before. 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.

Make sure you use the same colour that is in your tank already. If you can't identify it by colour alone, check your vehicle handbook. From there, you can buy whatever coolant meets your budget.

Here's how to check your coolant tank.


How to Check and Top Up Engine Coolant

  1. Make sure the car is cold before you open the bonnet. Complete this check before you travel or at least 30 minutes after you switch the engine off.
  2. Open the bonnet and find the coolant tank. It is transparent, usually sits in the centre of your engine bay and will have a bright coloured liquid in it. Your vehicle handbook shows its exact location if you're struggling.
  3. You should be able to see the level and colour without removing the radiator cap. The liquid should touch the maximum line on the side of the tank or be just below it.
  4. If it isn't, you can top your tank up with a mixture of coolant and de-ionized water.


You should only top up if the level has dropped a little bit and there is no leak present. Your car doesn't consume coolant as it drives along, so a very low level is bad news. 


How Do I Know if I Have a Coolant Leak?

There are several ways that you can identify a coolant leak. You may see a light-coloured stain around the radiator cap, on the hoses in your engine bay and/or on the engine itself. If the hose is leaking near a clamped end, you can tighten it with a screwdriver. 

You can also check for a puddle under the car after it has been parked for 15 minutes. It will be the same colour as the coolant in your engine bay and smell sweet. If you don't see a puddle but notice the smell, look under the car to see if anything is wet.

If there is a puddle, open the bonnet to try and find the source. You should also sniff around to find where the smell is coming from. If it is stronger inside the vehicle than outside, the leak could be in the heater. 

Whether you can pinpoint the leak or not, you must take these signs seriously. You should book an appointment with an expert as soon as possible to confirm where the leak is. They can then fix it for you with a coolant change.



What Does the Coolant Warning Light Look Like?

If you ignore a leak and continue driving without coolant, your engine will begin to overheat. This will then switch on the coolant warning light, which looks like this:


red thermometer above two wavy lines on black background representing coolant warning light


If you can see it on your car's dashboard, you MUST stop as soon as it is safe to do so. The cooling process has failed and the engine temperature is on the rise. If you let this continue, you can cause permanent damage which will need expensive repairs.

A coolant leak is the most common reason why the warning light comes on. While it could also be a sensor malfunction that will require a diagnostic check, you must seek expert advice from a mechanic before drawing any conclusions.

A coolant change is your best option. A mechanic will inspect your coolant, identify any problems, fix any leaks and add the right type of coolant all for an affordable price. That way, your car keeps driving smoothly and you avoid hundreds of pounds worth of engine repairs.