There are three main coolant chemistries: IAT (inorganic additive technology), OAT (organic acid technology) and HOAT (hybrid organic acid technology). Which one your car needs depends on the engine's metals and what the manufacturer specifies. It does not depend on the colour of the fluid. Get it right and the cooling system lasts. Get it wrong and you can end up with corrosion, sludge and an overheating engine.
This guide covers the chemistry in plain words, what the colours do and don't tell you, whether you can mix coolants, how to find the right product for your car in Australia, and what to do with the old fluid.
TL;DR
- Coolant comes in three main chemistries: IAT for older cast-iron engines, OAT for modern aluminium engines, and HOAT, a hybrid of the two.
- Colour is only a dye. It is not a reliable guide to the type, so read the label and your owner's manual.
- Don't mix different coolant types. Same-type products from different brands are generally fine, but check the specification first.
- Always buy by manufacturer specification, not by "universal" marketing claims.
- Used coolant is hazardous waste. Take it to a council-approved drop-off, never down a drain.
Table of Contents
- What coolant actually does
- The three main coolant types
- What coolant colour really means
- Can you mix coolants?
- What coolant does my car need?
- A quick checklist before you buy
- We see the same mistakes again and again
- Plan the job before you buy anything
- Getting rid of old coolant safely
- FAQ
- Sources
What coolant actually does
Most people call it antifreeze, but that undersells it. Engine coolant uses ethylene glycol to widen the temperature range of water, lowering its freezing point and raising its boiling point. The same fluid also carries additives that fight rust and corrosion inside the engine and radiator.
In Australia the freezing point matters less than the boiling point. Your engine sits in summer heat, in traffic, towing a van up a range. The coolant has to keep the engine at a stable temperature without boiling over, and protect the metals it touches while it does so.
Engineers describe it as a thermal management fluid, not just antifreeze. Beyond freeze and boil protection, it has to inhibit rust, prevent cavitation and control surface tension for heat transfer. Modern aluminium cylinder heads and heat exchangers are sensitive to chemistry. That's why the choice of additive package is the whole point, and why the colour is a sideshow.
The three main coolant types
The industry sorts coolants by their inhibitor technology. IAT uses traditional silicates and phosphates for older cast-iron engines, OAT uses organic acids for modern aluminium engines and lasts longer, and HOAT combines both for mixed-metal systems.
| Type | Inhibitors | Typically found in | Service life |
|---|---|---|---|
| IAT | Silicates and phosphates | Older cars with cast-iron engines | Shortest, changed most often |
| OAT | Organic acids (carboxylates) | Modern aluminium-heavy engines | Extended |
| HOAT | Organic acids plus a small inorganic part | Mixed-metal cooling systems | Extended |
IAT: the classic green
Inorganic additive technology is the old-school formula. The silicates and phosphates form a protective layer on metal surfaces quickly, which suits older cast-iron engines and radiators. The trade-off is that those additives get used up, so IAT coolant has the shortest service life of the three. If your car is from the era of simple iron blocks and brass radiators and the manual calls for conventional coolant, this is what it means.
OAT: long life for modern engines
Organic acid technology drops the silicates and phosphates. Instead of coating the whole system, the organic inhibitors target spots where corrosion starts, and they deplete slowly. As Shell's engine coolant guide explains, the increased use of aluminium in modern engines made this change in coolant technology necessary. The same source notes that some modern OAT and HOAT products are proven for 5 to 10 years of service. Local formulations exist too: one Australian-blended product is described by its maker as an ultra long-life organic coolant made with carboxylate technology inhibitors.
HOAT: the hybrid
Hybrid organic acid technology blends the two approaches. It keeps the long-life organic inhibitors and adds a smaller inorganic component for fast protection on certain metals. HOAT is aimed at mixed-metal systems, which is why you see it in many European and Asian vehicles. Some commercial products show how this plays out: one widely sold Australian-market concentrate is an ethylene glycol coolant containing a balanced mixture of organic acid and inorganic inhibitors.
OAT vs HOAT: what's the real difference?
Both are long-life chemistries. The practical difference is the additive package:
- OAT is purely organic. It's free of silicates and phosphates and is designed around aluminium-heavy engines.
- HOAT adds a small inorganic element to the organic base, giving a blend of both protection styles.
For a DIYer, the takeaway is simple. They are not interchangeable just because both say "long life". Your manual will name the specification, and the bottle must match it.
What coolant colour really means
This is the biggest trap for home mechanics. Green is IAT, orange is OAT, yellow is HOAT: that rule of thumb is half-remembered folklore. Colour is just a dye the manufacturer adds and doesn't reliably indicate the chemical type, so choosing by colour alone is unsafe. Check the label for the chemical type or the specific manufacturer approval.
There's a second reason colour misleads. Coolant is clear when it's made, and the dye is added afterwards to suit market preference or a manufacturer's spec. Two brands can sell the same colour with different chemistry, and the same chemistry in different colours.
So what is colour good for?
- Spotting leaks. A bright dye makes a drip on the driveway easy to see.
- A quick hint, not a verdict. If the reservoir is a different colour from what the bottle says, something has been mixed or topped up with the wrong product. Investigate before topping up.
- Checking condition. Brown, rusty or oily coolant is a problem regardless of what colour it started as.
If someone asks "should I use blue or pink coolant?", the honest answer is: whichever one meets your car's specification. The colour is secondary.
Can you mix coolants?
The short answer: don't mix different types, and don't assume a different brand means a different type.
Different chemistries. Mixing types such as IAT and OAT can cause chemical reactions that lead to sludge or gel, which can block cooling passages, damage seals and cause overheating. Mechanics also warn that mixing two different inhibitor chemistries can dilute the effectiveness of both, cancelling the corrosion protection each was designed to give.
Same type, different brand. Two coolants of the same type from different brands are generally safe to mix. Even so, a car with a specific manufacturer approval is best kept on a product that carries that approval.
"Universal" coolants. Some products are marketed as suitable for every vehicle. The Ford FAQ is blunt on this: Ford has no performance data for such products and advises against using them. Always put the manufacturer's specification ahead of a bottle's marketing.
What to do if you've already topped up with the wrong one
- Stop adding anything. Don't try to "fix" it with a third product.
- Note what's in there and what you added. Keep the bottles.
- Have the system drained and flushed with the correct coolant. Don't leave a mixed system running for months.
- If the engine is overheating, or the fluid looks like gel or sludge, stop driving and see a mechanic.
In a true emergency, where the level is low and you're stuck, topping up with a small amount of distilled water is often safer than adding an incompatible coolant. Treat it as a stopgap to get you somewhere safe, then have the mix corrected. Never open a radiator or reservoir cap on a hot engine: the system is under pressure and the fluid can scald you.
What coolant does my car need?
This is the question that matters, and it has a boring but reliable answer: the one your manufacturer specifies. Here's how to find it.
- Owner's manual. Look in the capacities and fluids section. It will give a specification or approval, not just a colour.
- Reservoir cap or engine bay labels. Some cars have the required fluid printed near the cap.
- Manufacturer's dealer parts desk. Give them your VIN and ask for the coolant spec for your exact build.
- The product label and data sheet. A good coolant lists the manufacturer approvals it satisfies. Check your car's maker is on it.
In Australia, coolants must comply with standards such as AS 2108-2004 or AS 2108.1-2004. The data sheet for one widely sold product also lists approvals across global manufacturers, including Ford, GM/Holden, BMW and Toyota. The point is to use a product approved for your vehicle's engine rather than relying on generic labels.
Concentrate or premix?
Coolant is sold as a concentrate or as a ready-to-use premix. In a concentrate, glycol makes up most of the bottle and the rest is water and the additive package... check the bottle's own dilution instructions before using it. Concentrate normally needs diluting, and the correct ratio is on the label. Premix goes straight in. If you're topping up, make sure what you add is the same strength as what's in the system.
How often should coolant be changed?
It depends on the chemistry and the maker's schedule. Traditional IAT coolants need replacing more often, while modern OAT and HOAT formulations can offer extended life, with some products proven for 5 to 10 years. Your logbook interval is the number to follow, and it overrides any figure on a bottle or in a general article.
A quick checklist before you buy
- Find the specification in your manual, not the colour in the reservoir.
- Match the chemistry type (IAT, OAT or HOAT) and the manufacturer approval.
- Decide on concentrate or premix, and check the dilution.
- Buy enough for a full drain and refill, plus a little for topping up.
- Line up a clean container for the old fluid and know where you'll drop it off.
We see the same mistakes again and again
Our platform exists because car owners keep running into the same problem: the information is scattered. Coolant is a perfect example. One tab for the right spec, one for prices, one for the tutorial, one for what a workshop would charge. We were founded by a mechanic with more than 25 years of experience, and the coolant jobs we hear about most share a few patterns.
People buy by colour. It's the most natural thing in the world, and it's the most common reason a cooling system ends up with the wrong fluid. Read the label.
People top up with whatever's on the shelf. A small top-up with the wrong type feels harmless. It rarely is, because the damage from mixed chemistries builds quietly.
People skip the flush. A coolant change is easy to do badly. Drain-and-fill alone leaves a lot of old fluid in the engine block. Whether you do a simple drain or a full flush, the goal is the same: end up with one correct chemistry in the system.
People forget the real cost of the job. The bottle is cheap. The true DIY cost includes the right coolant, a drain pan, funnel, hoses or clamps if needed, and a proper way to dispose of the old fluid. Against that sits a workshop quote, which also has labour. The honest question is whether the difference is worth your time and comfort level, and that depends on your car and your skills. For a simple coolant top-up or change on an easy-access engine, DIY is realistic for many beginners. For a job that involves bleeding air from a complicated system, you may prefer a professional.
Our view: a good DIY decision is a well-informed one. Know the spec, know the full cost, know your skill level, and then choose.
— Ryan
Plan the job before you buy anything
If you're weighing up a coolant change, DIYPro lets you start with one question: what's wrong with your car, or what do you want to do? You then see the full DIY cost against a typical workshop quote, with every part, tool and product listed. You pick a budget, value or premium tier for the job, and follow a tutorial matched to your skill level. It's built for Australian car owners, and it's designed to end the juggling of a dozen browser tabs.
We're currently building up access, so you can join the DIYPro waitlist to be among the first to use it.
Getting rid of old coolant safely
Used coolant is hazardous. It carries glycols, and after use it can hold heavy metals. It must not go down drains, onto the ground or into stormwater. South Australia's EPA lists engine coolant among pollutants that must be kept out of the environment, and the same guideline covers storing such liquids in secure, labelled containers.
For home use, the practical steps are:
- Drain the coolant into a clean, sealable container. A spare coolant jug is ideal.
- Label it, and keep it somewhere secure and out of reach of children and pets. Glycol is toxic, and pets can be attracted to it.
- Take it to a drop-off. Council community recycling centres and household chemical collection events handle this kind of waste. For example, Hornsby Shire Council's Community Recycling Centre accepts motor oils and other automotive liquids from local residents, with proof of residency required. Victorian councils promote programs such as Detox Your Home, which collects common harmful household chemicals.
- Check what your local site accepts before you go. Rules and accepted items differ between councils and states, so confirm with yours.
Don't put coolant in a kerbside bin. Treat it like used engine oil.
FAQ
What are the three types of coolant?
They are IAT (inorganic additive technology), OAT (organic acid technology) and HOAT (hybrid organic acid technology). IAT suits older cast-iron engines, OAT suits modern aluminium engines with longer life, and HOAT combines both. Your manual's specification tells you which your car needs.
Does it matter what kind of coolant I use?
Yes. The additive package protects the specific metals in your cooling system, and the wrong chemistry can leave them under-protected. Mixing incompatible types can also form sludge that blocks passages. Always match the manufacturer's specification.
Should I use blue or pink coolant?
Choose by specification, not colour. Dye doesn't reliably indicate the chemistry, so two blue or two pink products can be completely different. Read the label for the type and for approvals that match your car.
Can I top up my coolant with water?
In a genuine emergency, a small amount of distilled water is safer than adding an incompatible coolant. It dilutes the mixture, though, so it's a short-term fix to get you somewhere safe. Have the system corrected with the right coolant soon after.
Is a coolant change a good DIY job?
For many cars with easy access, it's a manageable job for a careful beginner, provided the engine is cold and you have a plan for disposing of the old fluid. If your car has a complex system or you're unsure about the spec, ask a mechanic. A cost comparison of DIY against a workshop quote helps you decide.
Sources
- Just Chill: Making Sense of the Coolant Conundrum, Hagerty Media
- Common coolant types and their uses, Tark Solutions
- Antifreeze Buying Guide, AutoZone
- Can You Mix Coolant Brands?, AutoZone
- Understanding Engine Coolant Types, Shell Rotella
- Proguard Green Concentrate data sheet
- Premium Cool Plus product datasheet
- What Happens When You Mix Coolants, Valvoline Global
- Engine Coolant Frequently Asked Questions
- Auto Recyclers Guideline, EPA South Australia
- Community Recycling Centre, Hornsby Shire Council
- Detox Your Home, City of Whittlesea