You pop the hood, look at the expansion tank and see a purple fluid. The shop offers you green because it's "the same thing". An hour later you're wondering if you've just ruined your radiator. This looks like a small detail, but in practice it's one of the most common reasons a water pump and radiator give up early. Here's how it actually works.
What coolant is and what it actually does
Coolant, properly called engine coolant, isn't just "water that doesn't freeze". It's a mix of glycol, water and an additive package. The glycol lowers the freezing point and raises the boiling point. The additives are there to stop corrosion of the metals inside the engine, prevent scale build-up and stop the fluid from foaming.
The key thing to understand is that the additives wear out over time. The fluid can still look fine while the corrosion protection has already failed. That's where problems with the radiator, water pump and thermostat housing start.
G11, G12 and G13: what the labels really mean
The G11, G12 and G13 designations come from the VW group (Volkswagen, Audi, Skoda, Seat) and have become an informal standard the whole industry uses. The difference is in the additive chemistry, not the colour.
G11 is what's known as hybrid coolant, marked HOAT (hybrid organic acid technology). It contains silicates and some organic additives. A classic solution for older engines, roughly up to the mid-2000s, where the radiator had more brass and steel parts.
G12 and G12+ are organic coolants, marked OAT (organic acid technology). No silicates, with carboxylate additives. Developed for modern aluminium engines and longer service life.
G13 is essentially G12+ but based on propylene glycol instead of ethylene glycol, with more environmentally friendly additives. Newer VAG models, roughly from 2012 onward, come with G13 from the factory.
The point for the driver: the difference between G11 and G12 isn't marketing. They're chemically different fluids and mixing them causes problems.
Coolant colours and why they can mislead you
The biggest misconception in workshops and among drivers is that colour means specification. It doesn't. The colour is just dye the manufacturer adds so the fluid is visible in the system and can be told apart from oil or brake fluid.
In practice:
- Blue and green fluid is most often G11, but not always
- Pink, red and purple is most often G12 or G12+, but one manufacturer makes G13 in purple while another makes G12 in the same colour
- Yellow and orange vary by manufacturer
Two coolants of the same purple shade can have completely different chemistry. In other words, a driver picking by the colour in the expansion tank is playing roulette. In the workshop we always check the OEM specification from the service book first, not the colour in the tank. Colour often fools the mechanic too if they don't check.
What you can mix and what you absolutely shouldn't
The rules go from most dangerous to acceptable.
Ethylene glycol and propylene glycol must not be mixed. This is technically the most dangerous combination. The different chemical bases create deposits that clog the channels in the radiator and engine block. That means G13 (propylene) and a classic G12 or G11 (ethylene) don't go together, even if the colour looks similar.
G11 and G12 as a rule should not be mixed. The silicates in G11 react with the carboxylate additives in G12 and form gel or foam. The result is reduced flow through the radiator, overheating and accelerated water pump wear.
G12 and G12+ can usually be mixed with each other, but only if both meet the same OEM specification (say VW TL 774-D or newer). This isn't a rule to rely on if you're not sure.
Topping up with plain water in an emergency is allowed only as far as the nearest workshop. If your level dropped on the road and you have nothing else, top up with distilled water, or as a last resort tap water. You're diluting the additives and the freezing point, so it isn't a fix - it's a bridge to the workshop.
When you're in doubt about what coolant is already in the system, the safer option is to flush the system with distilled water and refill with new coolant of a known specification. It costs a bit more than a simple top-up, but it's the only way to be sure what's circulating in your engine.
How to know which coolant goes in your car
Three reliable sources, in order of reliability:
- Service book and owner's manual. That's where the OEM specification is written, usually as a code like VW TL 774, MB 325, BMW LC, Opel/GM or similar. That's what you look for on the coolant bottle, in small print on the back.
- Sticker on the expansion tank cap or under the hood. Many manufacturers print the specification there. Not a rule, but it helps.
- VIN number and a call to the authorised service or workshop. The VIN can be checked to see what the car got from the factory.
What doesn't work: asking the seller "do you have coolant for a Golf 5". Half of them will give you whatever they have on sale. Ask by specification, not by car model.
When to change it and what to check at every service
Additives have a service life, even if the fluid visually looks fine.
- G11 typically 2-3 years, depending on use
- G12 and G12+ typically 4-5 years
- G13 typically 4-5 years
These are rough ranges for average use. A car that often sits, does short trips or runs in heavy heat burns through additives faster. The manufacturer's recommendation from the service book always comes first.
At every service it's worth checking:
- Freezing point with a refractometer. A quick check, takes a few seconds and shows whether the fluid still protects against freezing
- pH value. A drop in pH below 7 means the additives have given up and the fluid is turning corrosive
- Visual inspection. Sediment, brown colour, oil on the surface or foam in the expansion tank are signs something is wrong
- Level and signs of leaks. Wet traces around the radiator, water pump, thermostat housing and hose connections. Cylinder heads and the intercooler on TDI/TSI engines are rarer but more serious causes
If you see the level dropping with no visible leak, an internal leak through the head or gasket is possible. That doesn't get fixed by topping up.
In short, coolant isn't a fluid you look at once every three years. Don't let the colour lead you - the designation and specification should. If you're unsure what's in your system or it's time for a change, drop by the workshop for a check. Better five minutes with a refractometer than a new water pump in six months.