How to check your car's coolant level before a summer breakdown
A five-minute check most drivers skip, until the temperature gauge tells them otherwise.
Coolant is not just water sloshing around your engine. It is a mixture of antifreeze and corrosion inhibitors that keeps your engine within its working temperature range in both a summer traffic jam and a freezing morning start. When the level drops too low, the engine loses some of its ability to shed heat, and on a hot motorway that can turn into an overheating warning within minutes. Checking it takes less time than filling up with fuel, and it can save you a roadside breakdown, or worse, a warped cylinder head.
The short version
Coolant is a roughly 50/50 antifreeze and water mix that manages engine temperature and stops corrosion, not just water.
Check it only with the engine cold, at the expansion tank, between the MIN and MAX marks.
Never open the radiator cap on a hot engine; the system is pressurised and can scald you.
Top up with the correct coolant type, and treat a repeatedly low level as a leak to investigate.
What coolant actually does inside your engine
A modern cooling system runs a roughly 50/50 mix of antifreeze concentrate and water, sometimes pre-mixed, sometimes not, through the engine block, radiator and heater core. That mix raises the boiling point of the liquid, lowers its freezing point, and carries additives that stop the aluminium and cast iron inside your engine from corroding from the inside out. Plain water alone boils sooner, freezes sooner, and does nothing to protect the metal, which is why topping up with water is only ever a temporary fix.
How to check the level without getting burned
Always check coolant with the engine cold, ideally before you have driven that day or at least thirty minutes after switching it off. Open the bonnet and find the coolant expansion tank, a semi-transparent plastic reservoir usually marked with MIN and MAX lines. Never open the radiator cap itself on a hot engine, the system is pressurised and can scald you. The level should sit between the two marks. If it is at or below MIN, it needs topping up before you drive any distance, particularly in hot weather when the system works hardest.
Topping up without making things worse
Check your owner's manual or the cap itself for the coolant type your car needs. They are not interchangeable, and mixing a long-life OAT coolant with an older IAT type can turn into a jelly-like sludge that blocks the system. Colour is not a safe guide here, since two coolants can share a colour and differ completely in chemistry, so match the manufacturer specification rather than the shade in the bottle. If you are using concentrate rather than a pre-mixed bottle, dilute it with distilled or demineralised water, not tap water, which carries minerals that encourage scale build-up. Top up slowly to the MAX line, refit the cap firmly, and check again after a short drive once the system has circulated.
Warning signs that mean more than a low level
A tank that is merely low once is normal evaporation over months. A tank that is empty again a week after you filled it points to a leak, not a top-up problem, and driving on regardless risks overheating the engine. Watch for a sweet smell from the engine bay, white or pale green residue around hose connections, a temperature gauge creeping toward the red, or the heater blowing cold when it should be warm. Any of those deserves a proper check before a longer trip, not after. It is the same five-minute discipline as checking your tyres properly before a long trip, the sort of habit that prevents a motorway stop rather than causing one.
Coolant matters in electric cars too
If you drive an EV or hybrid, do not assume you are off the hook, but do change what you check. Most electric cars still run one or more coolant loops, not for a combustion engine but to keep the battery pack, motor and power electronics within their ideal temperature range, which protects both performance and long-term battery health. Two things are different. The fluid is often a special low-conductivity coolant rather than ordinary antifreeze, because it sits close to high-voltage components, and the battery circuit itself is frequently sealed and meant to be serviced only by the manufacturer. So the reservoir you can safely check and top up is usually the one for the cabin heater or the power electronics, not the pack. Read the manual before you add anything, use only the exact coolant specified, and treat a level that keeps dropping as a job for the garage rather than the driveway. It is one more detail worth knowing alongside whether EVs really wear their tyres out faster. If you are shopping for your next car, you can also browse the latest listings on AutoNext.
Frequently asked questions
How often should I check my coolant level?
About once a month, and always before a long trip in hot weather. It only takes a minute once you know where the expansion tank is.
Can I top up with plain water in an emergency?
Yes, briefly, if you are stranded and have nothing else. Water will get you moving, but it dilutes the antifreeze and corrosion protection, so replace it with the correct coolant mix as soon as you can. Do not do this on an electric car's battery loop, where only the specified coolant belongs.
Why does my coolant need replacing if the level never drops?
The corrosion inhibitors in coolant wear out over the years even if no liquid is lost, which is why manufacturers set a replacement interval, often two to five years, regardless of the level. Check your service schedule for the exact figure.
What colour should my coolant be?
It depends on the type your manufacturer specifies. Pink, blue, orange and green are all used by different brands, and colour alone does not tell you the chemistry, since manufacturers do not dye to a common standard. What matters is that you never mix types, and that you match whatever the manual specifies rather than whatever looks similar.
