Quick answer
They are the main inhibitor technologies in engine coolant. IAT uses inorganic inhibitors (silicates, phosphates) and is the older, shorter-life chemistry. OAT uses organic acids and is long-life. HOAT is a hybrid of organic acids plus some silicate, and P-HOAT and Si-OAT are hybrid variants. Use the one your manual specifies, and avoid mixing different technologies.
What to buy
Start with the genuine coolant your car maker sells, if your owner's manual calls for it.
Have a spec code from your manual? Look it up in coolant specs explained.
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Confirm the specification in your owner’s manual before buying. Product examples are not a fitment guarantee.
Key takeaways
- The glycol base is similar. The inhibitor package defines the type.
- IAT: inorganic, conventional, shorter service life.
- OAT: organic acids, long-life.
On this page
The five coolant technologies compared
| Type | Inhibitors | Typical notes |
|---|---|---|
| IAT (Inorganic Additive Technology) | Silicates, phosphates | Conventional, older chemistry; shorter service intervals |
| OAT (Organic Acid Technology) | Organic acids (carboxylates) | Long-life; used widely by many carmakers |
| HOAT (Hybrid OAT) | Organic acids plus some silicate | Hybrid chemistry |
| P-HOAT (Phosphate HOAT) | Organic acids plus phosphate | Common in many Asian OEM specs |
| Si-OAT (Silicated OAT) | Organic acids plus silicate | Used in some European OEM specs |
These are general descriptions. Each coolant maker formulates differently within a family, which is why the OEM spec number matters more than the family name.
How inhibitors work
Inorganic inhibitors such as silicates form a protective layer over metal surfaces quickly, but they are used up faster. Organic acids protect more selectively, at the points where corrosion starts, and generally deplete more slowly, which is why OAT coolants are marketed as long-life. Hybrids aim to combine fast protection from a little silicate or phosphate with the longevity of organic acids. The exact service life is set by the product and the vehicle maker. Follow your manual's interval, as covered in how often to change coolant.
HOAT vs OAT: which is better?
Neither is better in general. The right one is the chemistry your engine was designed and tested with. Some engines and radiators benefit from a little silicate (HOAT) for fast aluminium protection. Others are specified silicate-free (OAT). Using the wrong one may not cause instant damage, but it can shorten service life or reduce protection over time.
Coolant mixing checker
Can you mix OAT and HOAT (or IAT)?
It's not recommended. Mixing dilutes each inhibitor package, and the combination may not protect as either coolant would on its own. In an emergency, a small top-up with the wrong type is better than running low. Afterwards, flush and refill to the correct spec. Check any combination below, or read can you mix coolant?.
How to identify the type in your car
- Read the coolant spec in your owner's manual.
- Check service records for what was last used.
- Read the reservoir cap and under-hood labels.
- If it's still unknown, assume it's unknown: drain and refill rather than mix.
Color won't tell you. See coolant colors.
European G-designations and how they map
In Europe coolants are often described with "G" names. Roughly, G11 is a silicate hybrid, G12/G12+ are OAT, and G12++/G13 are Si-OAT. G48 is a silicate hybrid. See the full guide: G11, G12, G13, G48.
Common mistakes
Common mistakes
- Choosing by color. Color isn't standardized.
- Assuming "long-life" means compatible. Long-life products can be OAT, HOAT or Si-OAT.
- Mixing technologies routinely. It dilutes protection. Use it only as an emergency top-up.
- Ignoring OEM spec numbers. The family name alone isn't enough.
Where ASTM D3306 fits
ASTM D3306 is the US standard specification for glycol-based engine coolant for automobiles and light-duty service. ASTM's summary says it covers ethylene glycol, propylene glycol and EG/glycerin coolants as concentrates and predilutes, with concentrates generally used at 40–60% in water. Its tests include freezing and boiling points, pH, chloride, corrosion, foaming and cavitation-erosion. It's a performance baseline, not a technology label. IAT, OAT and hybrid coolants can all be designed to meet it, which is why "meets ASTM D3306" doesn't tell you whether two coolants mix well.
Technology timeline
| Era | Common approach |
|---|---|
| Older vehicles | IAT: silicate and phosphate, shorter intervals |
| Long-life era | OAT: organic acids, extended intervals |
| Hybrids | HOAT, P-HOAT and Si-OAT blends tuned by carmakers |
Exact dates and adoption vary by carmaker and region.
Related tool
About to top up with a different coolant?
Check compatibility →Picking a technology when the manual is vague
- Look for an OEM spec number anywhere in the manual or service records.
- Ask a dealer for the factory fill specification by VIN.
- Choose a product that explicitly lists your make and model years.
- Avoid switching technology without flushing.
Common mistake
- Treating "long life" as a technology. It's a marketing term. Read the technology and spec.
What to do next
Know your technology, record it, and stick with it.
Write the technology (for example "P-HOAT") on a label near the reservoir. Anyone topping up later can match it, even if they don't know the spec number.
Use the mixing checker before any top-up with a different bottle.
Tip: When buying a used car, ask the seller which coolant was last used. If they don't know, budget for a flush.
When to see a mechanic
When to see a mechanic
See a mechanic promptly if…
- you've mixed coolant types and see sludge or deposits
- the system has overheated
- you want to switch technologies and need a thorough flush
Specs referenced: ASTM D3306 (glycol-base engine coolant for automobile and light-duty service) and ASTM D6210 (fully formulated glycol-base coolant for heavy-duty engines). Your owner's manual and the OEM spec number take priority.
Frequently asked questions
What is the difference between OAT and HOAT coolant?
OAT uses organic acid inhibitors only. HOAT adds a small amount of silicate to organic acids for faster metal protection.
What is IAT coolant?
Conventional coolant with inorganic inhibitors such as silicates and phosphates. It generally needs changing more often than long-life types.
Can I mix OAT and HOAT?
It's not recommended. Mixing dilutes both inhibitor packages. Flush and refill to your manual's spec if they have been mixed.
Which coolant type does my car use?
Check the owner's manual for the spec or technology. Color isn't a reliable guide.
Which technology is best?
The one your vehicle specifies.
Can I convert IAT to OAT?
Only if the carmaker permits it, and after a thorough flush.
Does technology affect color?
Not reliably.
Do all OAT coolants mix?
Same-spec OATs usually do. Others may not.
New to cooling systems? Start with how a car cooling system works. Looking for a product? Search the coolant directory or match your manual's spec with Find my car coolant. See also coolant by car make and coolants by brand.
Next step
Not sure which coolant to buy? Match the spec code in your owner's manual, then buy the coolant that lists it.
Find my car coolantWhere to buy coolantHow we source this page
- Facts come from manufacturer pages, datasheets, owner's manuals and standards bodies, linked under Sources.
- We do not test products, and no brand pays to be listed or ranked. Advertising and affiliate links are labelled and never change the data.
- Anything we could not find is marked "not published" rather than guessed. Spotted an error? Tell us.
Sources
Sources
- ASTM D3306: Glycol base engine coolant for automobile and light-duty service
- ASTM D3306: Glycol base engine coolant for automobile and light-duty service
Want every family on one page, including P-HOAT, Si-OAT and nitrited NOAT? Open the coolant types hub.