How to check your coolant and top it up

Coolant is the fluid almost nobody checks. Oil has a dipstick and a folklore, tyres have a legal limit and a scary headline, and brake fluid at least sounds like something that ought to be looked at. Coolant sits in a translucent plastic bottle at the side of the engine bay, quietly doing the single most expensive job in the car, and most drivers never touch it until steam is coming out of the bonnet.

That is a shame, because it is the easiest check on the car. There is no dipstick to misread and nothing to unscrew. The tank is see through and it has two lines moulded into the side of it. You look at it. That is the whole job, and it takes about fifteen seconds once you know which bottle you are looking at.

The reason it is worth those fifteen seconds is that the cooling system is the one place where a slow, cheap, boring fault turns into a catastrophic bill without ever warning you properly. A weeping hose costs very little to replace. The overheat it eventually causes can warp a cylinder head, and a head gasket repair runs into four figures on plenty of ordinary cars. The gap between those two numbers is the argument for looking in the bottle.

What coolant actually is, and why it is not just water

Coolant is a mix of antifreeze concentrate and water. The water is there to carry heat, because water is genuinely excellent at that. The antifreeze is there to fix the three things water is bad at.

It stops the mixture freezing in winter, which matters more than it sounds. Water expands as it freezes, and an engine block full of ice is an engine block being pushed apart from the inside. It raises the boiling point, so the system can run hotter without turning to steam. And, the part most people never hear about, it carries a corrosion inhibitor package that stops the aluminium, steel, brass and rubber inside the system eating each other.

That third job is why old coolant is a real problem even when the level is perfect. The inhibitors are consumed over time. Once they are spent, the fluid is still a liquid, still the right colour, still between the marks, and is quietly corroding the radiator, the water pump and the heater matrix from the inside. Coolant does not fail by running out. It fails by going flat.

The Highway Code puts fluid checks on a weekly footing rather than an annual one. Its maintenance annex tells drivers to check the fluid levels in the vehicle at least weekly, and separately tells you to make sure there are appropriate anti-freeze agents in the radiator and the windscreen bottle before winter.

The one rule: check it cold

Before anything else, this is the rule that matters, and it is the only part of this job that can actually hurt you.

A cooling system is pressurised on purpose. That is how it runs above 100 degrees without boiling. When the engine is hot, the fluid in that tank is above its atmospheric boiling point and is being held liquid by pressure alone. Take the cap off and you remove the pressure, and the whole lot flashes to steam and comes out of the hole at you. This is the classic way people end up in A and E with scalded hands, forearms and faces.

The official wording is blunt. GOV.UK's guidance on overheated engines says that if your engine overheats you should wait until it has cooled naturally, and that only then should you remove the coolant filler cap. There is no clever technique that makes it safe sooner. A rag over the cap is not a technique. Cracking it slowly is not a technique. Waiting is the technique.

In practice, cold means cold to the touch, not warm. On a car that has been driven hard, that is a couple of hours. The genuinely good habit is to check it first thing in the morning before the car has been started, which is also when the reading is most accurate, because the fluid has contracted and settled.

The good news is that you almost never need to open the cap to check the level. The tank is translucent. You are reading it through the plastic. The cap only comes off to top up.

How to check your coolant level, step by step

Park on level ground and switch the engine off. On a slope the fluid sits at an angle in the tank and you will misread it, which is the same reason checking your engine oil starts the same way.

Open the bonnet and find the expansion tank. It is a translucent plastic bottle, usually white or slightly cloudy, with coloured fluid visible inside it, and it sits to one side of the engine bay rather than in the middle. The cap almost always carries a warning symbol and often the words "do not open hot". If you are unsure which bottle is which, the coolant one is the one with brightly coloured fluid in it. Screen wash is usually a paler blue and its cap carries a windscreen symbol, and the brake fluid reservoir is smaller, sits higher up near the bulkhead and holds a pale straw coloured fluid. Our guide to checking your brake fluid covers telling that one apart.

Look at the side of the tank for two moulded lines, marked MIN and MAX, or sometimes low and full, or a pair of unlabelled notches. Read the level of the fluid against those lines with your eye level with the tank rather than looking down at it from above.

That is the check finished. If the level is between the two marks, close the bonnet and get on with your day.

If it is at or below MIN, and only once the engine is genuinely cold, turn the cap a quarter turn and pause to let any residual pressure hiss out before removing it fully. Top up to the MAX line, not above it. Overfilling is not dangerous in the way overfilling oil is, but the fluid expands a great deal as it warms, so a tank filled to the brim simply pushes the excess out of the overflow and leaves you with a puddle on the drive and a false alarm.

Refit the cap properly and make sure it clicks or seats fully. A loose expansion tank cap is a surprisingly common cause of a system that will not hold pressure, which shows up as a car that runs hot on a long climb and is fine everywhere else.

What the min and max marks are telling you

The level moving a little is normal. The level moving steadily is a message.

That last row is the one to take seriously. A creamy, mayonnaise coloured film in the coolant means the two fluids that should never meet are meeting. Paired with white smoke from the exhaust it is a fairly classic picture, and what the colour of your exhaust smoke means goes through how to tell steam from genuine trouble.

What to top up with, and why the colour is not the specification

Here is the mistake that costs people money: buying antifreeze by colour.

Coolant is dyed so that you can spot a leak and so that you do not confuse it with anything else. The dye is not a standard. Red, blue, green, pink, orange and yellow are marketing decisions, and two bottles of the same colour on the same shelf can contain completely different chemistry.

What actually matters is the inhibitor technology and the manufacturer approval printed on the label, along the lines of an OAT or Si-OAT specification and a manufacturer code such as G12 or G13. Your handbook names the specification your engine needs. That is the thing to match, and the colour is incidental.

The reason this matters is that the inhibitor packages are chemically different approaches to the same problem, and mixing two of them can drop the additives out of solution as a gel or a sludge, which then blocks the narrow passages in a radiator or a heater matrix. Manufacturers are measured about it rather than hysterical. Comma, for instance, notes that its product is miscible with other fluids but that for maximum protection the system should be drained, flushed and refilled. Which is the honest version: mixing will not detonate your engine on the spot, it quietly costs you the protection you paid for.

If you genuinely do not know what is in the system, top up with the correct specification for your car and mention it at the next service. If the fluid is an unknown mixture of several previous top ups, the right answer is a drain and refill rather than adding a sixth flavour to the cocktail.

One buying tip that is worth more than the colour: some antifreeze is sold with a bittering agent added specifically to discourage accidental drinking. Given what the next section but one is about, that is worth a few pence.

How much antifreeze to water?

Coolant is sold two ways. Ready mixed, sometimes labelled ready to use, is already diluted and goes straight in. Concentrate has to be mixed with water first, and putting concentrate in neat is a real mistake, because antifreeze on its own is a worse coolant than the mixture is. It carries heat away less well than water does.

The strength of the mix sets the freeze protection. Comma publishes approximate figures for its concentrate that show the shape of it clearly.

For a British winter, a 50:50 mix is the sensible default and is what most manufacturers specify. It gives protection far beyond anything the UK climate will produce, which sounds like overkill until you remember that the mixture is also doing the corrosion job, and a weak mix is a weak inhibitor dose as well as a weak freeze point.

Going stronger than about 60 per cent is not a bonus. Past that point you are steadily removing the water that does the actual heat transfer, and the mixture gets thicker and worse at its main job. Stronger is not safer beyond the specified ratio.

If you are mixing it yourself, use deionised or distilled water rather than tap water. Tap water in a hard water area carries dissolved minerals that deposit inside the system as scale, which is exactly the sort of slow furring up that kills a heater matrix. A bottle of deionised water costs about a pound. Getting your winter checks in order before the cold arrives is covered in our guide to getting your car ready for winter.

Can you just top up with water?

In an emergency, yes, and you should not feel bad about it. If you are at the side of a road with a low tank and a long way to go, water in the tank is better than no coolant in the tank, and the AA and RAC both treat it as a legitimate get you home measure.

The caveats are real, though. Water alone freezes, so a car topped up with water in November and left on a driveway through a cold snap can crack something expensive. Water alone has no inhibitors, so every litre of it dilutes the corrosion protection in what is left. And water alone boils sooner, so a system already running hot has less margin than before.

Treat it as exactly what it is, a way of finishing the journey. Get the mixture corrected properly afterwards, and treat the fact that you needed it as the real finding: a healthy sealed system does not need topping up on the hard shoulder.

Why is my coolant disappearing?

This is the question behind almost every coolant check, and the answer is worth stating plainly. A cooling system is sealed. It is not like oil, some of which is genuinely consumed as the engine runs. If the level keeps dropping, the fluid is going somewhere, and there are only so many places it can go.

The pattern to notice is that the first three are ordinary, affordable jobs and the last one is not. They are also connected, because a slow leak that never gets looked at is one of the standard routes to an overheat, and an overheat is one of the standard routes to a head gasket. Our guide to why cars overheat covers what to do at the roadside if it gets that far.

One more symptom worth knowing, because it seems unrelated: a heater that blows cold, or only gets warm at speed, is a classic low coolant sign. The heater matrix sits at the top of the system, so it is the first thing to be starved of fluid when the level drops. If the cabin has gone lukewarm, check the tank before you assume the heater has failed, and why your heater is blowing cold goes through the rest of the causes.

A coolant leak will pass its MOT

This one surprises people, including people who have owned cars for decades, and it is the single most useful thing on this page.

The MOT does check for fluid leaks. Section 8.4.1 of the DVSA inspection manual covers them, and the threshold is specific: a leak that forms a pool over 75mm across within five minutes is a failure. Fluid leaking excessively and likely to harm the environment or pose a safety risk is graded Major, and fluid leaking continuously and likely to pose a serious risk to road safety is graded Dangerous.

Coolant is then explicitly carved out of it. The same section states that a leak of fluids such as engine coolant, screen wash and the fluid required for Selective Catalyst Reduction are not reasons for failure.

So a car can drip coolant onto the floor of the test bay, in front of the tester, and still pass. Not because the tester missed it, and not because it does not matter, but because the MOT is a roadworthiness test and a coolant leak is a reliability and engine damage problem rather than a road safety one.

The practical consequence is the bit worth carrying away. A fresh MOT certificate is not a statement that your cooling system is sound, and a great many people treat it as one. The MOT is a floor, not a health check. The place a cooling system actually gets inspected is a service, where the level, the condition of the fluid, the hoses and often the freeze protection itself are all checked. That is one of several things in what is included in a full service that the test simply does not look at, and it is a decent argument for booking an MOT and a service together rather than treating the certificate as the annual once over.

Antifreeze is lethal to cats and dogs

This is not a footnote, and it is left out of almost every article on topping up coolant.

Ethylene glycol, the base of most antifreeze, tastes sweet. Animals will drink it voluntarily from a puddle on a driveway or lick it off their paws and fur after walking through a spill. Cats Protection is unambiguous that even the smallest amount of antifreeze can be fatal to a cat.

The timeline is fast and deceptive. Signs can appear from about 30 minutes to 12 hours after ingestion, and include drooling, vomiting, appearing depressed, wobbliness and increased thirst. Left untreated, kidney damage worsens after 12 hours, and by around 24 hours the kidneys can fail entirely with a risk of death. Treatment works best when it starts within about eight hours of exposure, which is often before an owner has any idea anything has happened.

What that means in practice when you are topping up on the drive:

• Clean up every spill and drip properly, straight away, rather than hosing it into a corner of the driveway where it will pool.
• Never leave drained coolant standing in an open bowl or tray while you get on with something else. It is the most attractive thing in the garden to a cat.
• Keep full and empty containers shut and out of reach. An empty bottle still holds enough.
• Take old coolant to a household waste recycling centre rather than putting it down a drain.
• Treat the puddle under your own car as a hazard to your neighbours' animals as well as a repair to book.
• Buy antifreeze with a bittering agent where you have the choice.

If you think an animal has drunk any, it is a same hour emergency vet call, not a wait and see.

How often should coolant be changed, and what does it cost?

Level checking and coolant changing are two different jobs, and this is where the inhibitor point from earlier pays off.

Modern long life coolants are typically rated at around five years. Older conventional types were nearer two. Manufacturers vary a lot, and some now specify a lifetime fill that in practice means a very long interval rather than never, so your handbook or your garage's schedule is the answer for your car rather than a rule of thumb.

A drain and refill at an independent garage is an inexpensive job. The work itself is typically well under an hour, and UK independents commonly charge somewhere in the region of £40 to £80 an hour, so with fluid most drivers should expect to pay roughly £50 to £120 depending on the car and the specification of coolant it takes. Cars with awkward bleeding procedures, and anything needing a specialist long life fluid, sit at the higher end.

Set against the parts it protects, that is one of the better value jobs in motoring. A radiator, a water pump and a heater matrix are all several times the price, and a head gasket is an order of magnitude more. It is also usually cheapest to have it done as part of a service you were booking anyway, which our guide to what a car service costs and how often you should service your car both cover.

A good independent will also test the freeze protection with a refractometer rather than guessing, which takes seconds and tells you whether the mix is still doing its job. It is a reasonable thing to ask for, and one of the questions worth asking a garage before booking.

When to stop driving and call a garage

Most coolant findings are a booking, not an emergency. These are the ones that are not.

The amber and red distinction on the dashboard is worth knowing here, because coolant has both. An amber coolant symbol generally means the level is low and wants attention. A red one means the engine is overheating or the level has fallen below its minimum, and that is a stop now warning rather than a book it in one. Our guide to what the dashboard warning lights mean covers the full set.

Otherwise, this is a fifteen second check that you can fold into the same routine as checking your tyre tread, and it belongs on the list every time you are about to do a long run, which is why it features in checking your car before a long drive. Look in the bottle once a month. If it keeps going down, that is the fault talking, and finding it while it is still a hose is the entire trick. Choosing somewhere you trust to do that is covered in how to choose a good garage.