A guide to engine coolant

Parkers explains what coolant is, what it does, how to get the correct coolant for your car, and how to replace it.

An engine coolant reservoir

by Chris Williams |

To the uninitiated or the disinterested, coolant may seem a complex smorgasbord of codes and colours but fear not, Parkers is here to make things simple. We’ve covered topics from AdBlue to bike racks. Now it’s the turn of coolant.

What is coolant?

Coolant is a 50-50 mix of antifreeze (ethylene glycol) and water. The antifreeze prevents the water from freezing, it also prevents overheating by increasing the boiling point of the water.

Though the idea is universal, coolants aren’t. Different cars require different coolants, though it’s not difficult to find which one you need for your car, which we will get into presently. The type of coolant largely depends on a car’s age. Pre-2000 vehicles often use what’s called Inorganic Additive Technology (IAT), while later models use Organic Acid Technology (OAT) and Hybrid Organic Acid Technology.

OAT coolants improved on IAT products by offering better protection and allowed for extra mileage between changes. In individual circumstances, it pays to refer to your car’s handbook. It will tell you not only the type of coolant you need but how often it needs changing.

Buying the right coolant

There are options here. Dealerships usually stock coolant for their models and you can visit them to get more. Alternatively, there are quality aftermarket options too available for many (but not all) car models. Whatever your choice, it’s vital to know that such products are available both as a ready-mix or and concentrate. The former can be poured straight into the reservoir. The latter requires diluting first but is much better value.

If you’re buying aftermarket coolant and you don’t have your car’s handbook, Halfords has the best coolant search by model. Simply type in your car’s registration number or model details and the correct coolants are filtered.

But as a time-saver, below are coolant options for some of the UK’s most popular vehicles, a Mk7 Ford Fiesta 1.0-litre petrol EcoBoost, a 2021 Nissan Qashqai 1.7-litre diesel, and a Fiat 500 1.2-litre petrol.

Comma XSR5L Xstream G30 Antifreeze and Coolant Concentrate - 5L

Comma XSR5L Xstream G30 Antifreeze and Coolant Concentrate - 5L
Amazon

View offer

Five-litre concentrate gives you excellent value. Ideal for use if you wish to flush the cooling system.

Comma XSM5L Xstream G30 Antifreeze and Coolant Ready Mixed - 5L

Comma XSM5L 5L Xstream G30 Antifreeze and Coolant Ready Mixed
Amazon

View offer

Ready to be poured straight into the coolant reservoir.

Prestone

Contradicting what we said before about coolants not being universal, Prestone claims to have a universal coolant. It’s highly rated by many customers and comes with a 10-year guarantee for corrosion resistance. It stands as an exception and feel free to use it, though model-specific coolants are likely to be more effective.

Prestone Ready to Use Coolant Antifreeze - 4L

Prestone Ready to Use Coolant Antifreeze - 4L
Amazon

View offer

A universal coolant for use on any model of car. It's unique in this respect.

When and where to top up

Coolant is not something that needs changing regularly. Check your vehicle’s manual for exact mileage but cars do tens of thousands of miles between coolant changes. Symptoms of low or old coolant in the system are overheating or the coolant warning light on the dashboard illuminating.

Coolant does not get poured into the radiator. It gets poured into the reservoir labelled ‘coolant’ and is connected to the radiator by a hose. When the engine is old, open the coolant reservoir and check the coolant is at the cold fill line. If it's low, it’ll need topping up. If the coolant in the reservoir is murky or gloopy you might want to consider flushing the system and filling the cooling system with fresh coolant. Remember, coolant is not just about regulating the car’s temperature, it also helps protect against engine corrosion.

Flushing the cooling system removes the build-up of deposits and particles that build up the hoses. Getting rid of this avoids the risk of damaging engine parts. Flushing can be beneficial for buyers of second hand cars too. In addition to removing sludge build-up, you may not know what coolant is in there, and since you can’t mix different types of coolant, it may give you peace of mind to flush the system.

How to flush the cooling system

  1. Ensure the engine and current coolant are cold. Never flush with a warm engine.
  1. Made easier with a car jack, locate the radiator drainage point on the underside of the car.
  1. Place a receptacle beneath the drainage point and open the valve, which may require a ratchet.
  1. Let the coolant drain and tighten up the valve again. Pour in system flush (see below) and water into the reservoir up to the fill line.
  1. Run the engine for about 10 minutes and allow it to completely cool before draining the system like you did before. There will likely be plenty of gunk that comes out this time.
  1. Refill the system with fresh coolant.

Holts Speedflush - 250ml

Holts Speedflush - 250ml
Amazon

View offer

An excellent, reliable solution that effectively cleans a radiator and cooling system.

Want to know how to change your car’s oil or washer fluid? Check out our videos below.

Read next:

The best degreaser for cleaning your engine and car

Rust converters and rust repair: Parkers' 'how to' guide

Tyre sealant: Parkers' guide

Sign up to the Parkers Newsletter to keep up to date with more of the latest reviews, news, and recommendations from the Parkers team.

Just so you know, whilst we may receive a commission or other compensation from the links on this website, we never allow this to influence product selections - read why you should trust us