How to Check and Top Up Coolant Without Guessing the Colour

Colour is marketing, not chemistry

Coolant used to be green. Then General Motors introduced orange Dex-Cool in the mid 1990s, other manufacturers picked their own dyes to signal “different formula”, and now the shelf at Halfords looks like a paint chart. Pink, orange, blue, yellow, red, green, purple, turquoise. The dye is dye. It tells you what the marketing team wanted the bottle to look like on the shelf. It does not tell you which corrosion inhibitors are inside, and those inhibitors are the entire reason the fluid exists.

Two different formulations can share a colour. Two identical formulations can be sold in different colours by different brands. Mix the wrong two families and the additives drop out of solution as a brown gel that clogs the radiator, the heater matrix, and the thin passages inside an aluminium block. Ford put out a technical service bulletin years back on gelled coolant blocking heater cores after somebody topped an orange OAT system with green IAT. A new heater matrix on a Focus is a dashboard-out job, roughly £500 in labour before parts.

The three families that actually matter

Almost every antifreeze on a UK shelf falls into one of three chemistries:

IAT uses silicates and phosphates that coat the metal directly. Fast acting, short lived. OAT uses organic acids (usually 2-EHA or sebacate) that only work where corrosion is starting. Slow acting, long lived, and hostile to some older rubber seals. HOAT is a hybrid, silicates for quick protection plus organic acids for longevity. There are sub-groups too, P-HOAT with phosphates for Toyota, Honda and Hyundai, and Si-OAT for VAG G12++ and G13. For a top-up decision on a typical UK car, the three main families cover it.

Check the level without burning your hand

Coolant lives in a sealed loop under pressure. Open a hot cap and superheated fluid flashes to steam in your face. Wait until the engine is stone cold. First thing in the morning is ideal.

1. Park on level ground. Bonnet up. Find the expansion tank, a translucent plastic bottle usually near the front of the bay with MIN and MAX marks on the side.
2. Read the level against the marks with your eye at tank height. Do not squeeze it, do not open it, just look.
3. If the fluid sits between MIN and MAX, you do not need to add anything. Close the bonnet.
4. If it is at or below MIN, or the tank is bone dry, you have a top-up or a leak on your hands.
5. If you can only check when warm, the level reads slightly higher than it will when cold. A tank reading MIN warm may be under MIN cold.

The dipstick-and-radiator-cap method belongs to older cars without an expansion tank. If yours has one, do not open the radiator cap at all. It is designed to stay pressurised, not to be fiddled with.

Work out what is already in the system

Before you add anything, find out what should be in there. Three sources, ranked by how much I trust them:

• The owner handbook. The fluids section gives a specification, usually a manufacturer code. Ford WSS-M97B44-D, VW TL-774 G/J, Toyota SLLC, BMW GS 94000. This tells you the chemistry family and often the exact approved brand.
• The service book or the last invoice. If a garage flushed and refilled, they wrote down what they used. Look for the product name.
• The colour in the tank. Last resort. Better than nothing but frequently misleading, particularly on a car whose previous owners topped up with whatever was cheapest that afternoon.

If none of that helps, the ASTM D3306 standard is the baseline spec for automotive coolant, and BS 6580 is the UK equivalent. Any bottle that meets those and matches the family (OAT if the car needs OAT) will not attack the metal. Prestone claims cross-family compatibility on its universal range, and I have used it as a get-me-home top-up, though for a full refill I still match the original spec. Their compatibility notes are on the Prestone site at https://www.prestone.co.uk if you want the official line.

Top up without gambling

Once you know what is in there, the process is dull, which is how it should be.

1. Buy the correct family in a concentration you can pour straight in. Most modern bottles are pre-mixed 50/50 with water. A concentrate needs diluting with deionised or distilled water, never tap water in a hard-water area (most of the South East and East Anglia in the UK, plus pockets of the Midlands), because the limescale plates out on the hottest parts of the block.
2. With the engine cold, twist the expansion tank cap off slowly. You may hear a small hiss as any residual pressure equalises. No hiss is fine too.
3. Pour slowly up to the MAX line. Do not overfill. The tank needs air space for expansion, which is what the marks are for.
4. Cap on, hand tight plus a quarter turn. Start the engine, let it run to full operating temperature with the heater on hot, watch the level. It may drop as trapped air works its way out. Top up once more when cold if needed.
5. If the level drops again a week later, you have a leak. White crust round a hose clip, a sweet smell in the cabin, a puddle on the drive, or unexplained pressure loss. Book it in rather than keep topping up.

Never add plain water except in a genuine emergency, and only enough to get home. Coolant is antifreeze plus water plus inhibitors, and the ratio matters. Too much water and it freezes at 0°C, boils at 100°C, and the inhibitors are too dilute to protect the metal properly.

When to flush instead of top up

Sometimes the honest answer is not “add more” but “drain the lot”. Flush and refill if any of the following applies:

• You genuinely cannot tell what is in the tank, or the colour looks brown, rusty, or has floating bits.
• The service interval on the current fill has passed. OAT at 5 years is not a rule to bend, the organic acids deplete whether the car sits on the drive or not.
• Somebody previously added the wrong family and the coolant looks cloudy or has any sludge on the underside of the cap.
• You just bought a used car with no service records.

A DIY drain-and-refill on most cars takes 2 hours, about £15 in coolant for a 5-litre system, and a plastic tray from a pound shop to catch the old fluid. Old coolant goes to the council waste site, not down a drain, because ethylene glycol is toxic to pets and wildlife (a teaspoon can kill a cat). The RAC walks through the process at https://www.rac.co.uk/drive/advice/car-maintenance/how-to-check-and-change-your-cars-coolant/ if you want to see one worked through.

Related reading

• The Quiet Habits That Make a Car Last 20 Years
• The Honest Guide to Washing Your Car Without Damaging the Paint