How to Check Your Oil Level
The other five-minute under-the-bonnet check: reading the dipstick correctly and knowing when to top up.
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Topping up coolant is one of the easiest jobs under the bonnet, with one serious rule: never open the cooling system while the engine is hot. The coolant inside is under pressure and can spray out hot enough to burn you. Wait until the engine has cooled down, and the rest is simple. Most people who search for this have just seen a warning light, or been told their coolant is low, and worry about two things: opening the wrong cap and buying the wrong bottle. This guide deals with both. You'll find the right tank, read the level, choose the coolant your car actually needs (the colour won't tell you), top it up, and learn when a level that keeps dropping means a garage, not another bottle.
What you'll be able to do: Check your car's coolant level safely, choose the correct coolant for your car, top it up yourself, and recognise when low coolant is a sign of a leak that needs a mechanic.
Outcome: "I can find my car's coolant reservoir and read the level correctly, with the engine cold and without opening anything."
With the engine cold (parked for a few hours, or first thing in the morning), open the bonnet and find a translucent plastic tank with MIN and MAX marks on the side. Don't open it yet. Just look at where the liquid sits.
The coolant reservoir, also called the expansion tank, is a translucent plastic tank in the engine bay. You can see the liquid level through its side, between two marks: MIN and MAX, LOW and FULL, or COLD and HOT. Its cap usually carries a warning symbol or a temperature warning. If you're not sure which tank it is, the owner's manual has a picture of the engine bay. The washer-fluid tank, often blue, with a windscreen symbol on the cap, is a different tank. Always check with the engine cold. Hot coolant expands, so a reading after a drive is higher than the real level. More importantly, the system is under pressure when hot. Opening the reservoir cap or the radiator cap on a warm engine can release a spray of hot coolant and steam. Let the car cool for at least 30 minutes, and preferably a few hours. The cap should feel cool to the touch before you go near it. A level between the two marks is fine. Below MIN, it needs topping up. If you can't see any coolant at all, or the temperature warning came on while driving, don't drive further than you must, and read Chapter 3 before adding anything.
A driver's dashboard shows a coolant warning on a cold morning. They open the bonnet and find a large blue-capped tank, nearly empty, and are about to fill it. Checking the manual's engine-bay picture first, they see that tank is the windscreen washer reservoir. The coolant tank is a smaller translucent one near the radiator, with a cap marked with a warning symbol, and its level sits just below MIN. They now know what to buy and where it goes, without having opened anything.
Outcome: "I know which coolant my car needs, whether to buy ready-mixed or concentrate, and what to do if all I have is water."
Look in your owner's manual for the coolant (or "antifreeze") section, and write down the exact specification it gives. It's usually a type or a manufacturer code, not a colour.
Coolant comes in several chemical types, and car makers specify which one their engines need. The owner's manual names it, sometimes as a manufacturer code, sometimes as a technology such as OAT or HOAT. Colour is not a reliable guide. Different brands dye the same type differently, and the same colour can be a different type. Buy the coolant that matches the specification in your manual, or ask the parts counter to match it to your make, model and year. Mixing types is best avoided, because it can reduce the protection against corrosion. If you don't know what's in the car now, stick to the manual's specification for top-ups and mention it at the next service. You'll see two forms on the shelf. Pre-mixed (often labelled 50/50) is ready to pour in, which is the easiest choice for a top-up. Concentrate has to be mixed with water first, usually half and half with distilled water, following the label. If you're stuck with an empty tank and no coolant, plain water will get you to a garage or a shop. But water alone doesn't protect against freezing or corrosion, so have the mix put right soon afterwards. Coolant is poisonous and tastes sweet, which makes it dangerous to pets and children. Wipe up any spill and keep the bottle closed and out of reach.
At the parts shop, someone reaches for a green bottle because the coolant in their tank looks green. The manual, though, specifies a particular long-life type by its code. The green bottle on the shelf is a different, older type. The counter staff look up the car's make, model and year and hand over a pre-mixed bottle that matches the manual's code, in a different colour. The driver buys that one, and keeps the receipt in the glovebox with the specification written on it.
Outcome: "I can top up the coolant to the right level, close the system properly, and tell whether a dropping level needs a mechanic."
With the engine cold, put a rag over the reservoir cap and turn it a quarter turn, slowly, then stop for a few seconds before taking it off. That pause is the habit that keeps you safe every time.
With the engine cold, cover the reservoir cap with a rag and open it slowly, pausing after the first turn so any remaining pressure can escape. On most modern cars, the reservoir is where you add coolant. Older cars may have a radiator cap as well, but follow your manual and don't open the radiator unless it says to. Pour the coolant in slowly until the level reaches the MAX (or FULL, or COLD) mark, never above it. The coolant needs room to expand when it gets hot. Close the cap firmly until it clicks or stops, wipe any spills, and close the bonnet. Check the level again over the next few days, always cold. A small top-up that stays put is normal over time. A level that keeps dropping is a different matter: the coolant is going somewhere. Look under the car for drips or a coloured puddle where you park, and watch for a sweet smell, steam, or the temperature gauge rising. Any of those means a leak or another fault, and it needs a mechanic, not more top-ups. If the temperature warning comes on while you're driving, pull over when it's safe, switch the engine off and let it cool. Don't open the cap while it's hot. Refilling a system that has been fully drained also needs the air bled out of it, which is a job for the manual or a garage.
A driver tops up their coolant to MAX on Saturday morning. By Wednesday, checked cold, it's back below MIN, and there's a small orange spot on the driveway under the front of the car. Instead of buying another bottle, they book the car in and mention the puddle and the dates. The garage finds a split hose and replaces it. The driver checks the level twice more over the next week, cold, and it stays at MAX.
The complete guide — all 3 chapters, every checklist and exercise — as a clean, printable PDF. No email required.
Every guide on this site is produced with the Self-Directed Learning Framework: instead of an article that explains a topic at you, the framework turns it into a structure you work through — each chapter with one outcome, a starting action under twelve minutes, the minimum you actually need to know, a worked example, a checklist, and exercises. It starts from three questions:
Topic: Topping up a car's coolant.
Outcome wanted: Check and top up the coolant safely, with the right product, and know when a low level needs a garage — within three days.
Starting point: Drives every day, has never opened the bonnet for more than washer fluid, and just saw a coolant warning.
The prompt framework behind it is public: see the C.R.A.F.T. prompt framework, including the exact prompt that generated this library.
The other five-minute under-the-bonnet check: reading the dipstick correctly and knowing when to top up.
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