Engine coolant (antifreeze) specification defines the chemical formulation that protects your cooling system from freezing, boiling over, and internal corrosion. Different formulations use different corrosion inhibitor chemistries matched to the specific metals in your engine and cooling system. Mixing incompatible types triggers a chemical reaction that creates deposits, damages seals, and accelerates corrosion from the inside.
Quick answer
Your coolant type is in the owner’s manual under “Coolant” or “Fluids.” There are three primary types: OAT (orange, red, or pink — used by most Asian brands and GM), HOAT (yellow or turquoise — used by Ford, BMW, Mercedes, and Mopar), and IAT (traditional green — pre-2000 vehicles). Never mix types. Top off with pre-mixed 50/50. Check freeze protection with a refractometer — target is -34°F.
Specifications
| Coolant Type | Color | Change Interval | Chemistry |
|---|---|---|---|
| OAT (Organic Acid Technology) | Orange, Red, Pink | 5 yr / 150,000 mi | Carboxylate inhibitors |
| HOAT (Hybrid OAT) | Yellow, Turquoise | 3–5 years | Carboxylate + silicate |
| Si-OAT (Silicated OAT) | Purple, Violet, Blue | 5 years | OAT + silicates (VW G13) |
| IAT (Inorganic Additive) | Green | 2 yr / 30,000 mi | Silicates + phosphates |
| Standard mix ratio | 50% coolant / 50% distilled water → protects to -34°F (-37°C) | ||
| Max freeze protection | 70% coolant / 30% water → -84°F (not recommended — reduces heat transfer) | ||
How to find your spec
- Owner’s manual — look under “Coolant,” “Antifreeze,” or “Engine Fluids.” Lists the approved specification by chemical type (OAT, HOAT) or OEM part number.
- Coolant reservoir label — many manufacturers print the required coolant type directly on the reservoir cap or on a label affixed to the reservoir itself.
- Under-hood sticker — some vehicles (especially GM) have a coolant type warning label near the reservoir identifying the type and warning against mixing.
- What’s in the system already — color gives a clue, but not a guarantee. Previous owners may have added the wrong type. If the history is unknown, test with a refractometer and look for cloudiness or unusual color. When in doubt, flush and start fresh with the correct type.
- OEM part number lookup — manufacturer part numbers for coolant concentrate (e.g., Motorcraft VC-7-B, DEX-COOL, Toyota SLLC) confirm the correct chemistry. Cross-reference these against aftermarket equivalents that carry the same specification approval.
How to check
- Check when cold — never open the radiator cap or reservoir on a warm engine. Pressurized hot coolant exits fast and causes serious burns.
- Check the reservoir level — should sit between MIN and MAX marks.
- Check concentration — place one drop of coolant on a refractometer prism, close the cover, read through the eyepiece. Target: -34°F protection or better for most US climates.
- Check color and clarity — healthy coolant is bright and clear. Murky, rusty, or oil-contaminated coolant needs a full flush before topping up.
- Top off with the correct pre-mixed 50/50 type — never add water alone in freezing climates, and never add straight concentrate undiluted.
What goes wrong
- Mixing OAT and IAT — the different inhibitor packages react and produce a gel-like precipitate. It coats the heater core, clogs the water pump impeller, and settles around the thermostat housing. Full flush plus heater core replacement is common: $500–$1,200.
- Topping off with tap water — acceptable as a one-time emergency to get home. Mineral deposits from tap water form scale on aluminum surfaces. Flush with distilled water as soon as possible afterward.
- Ignoring the change interval — the ethylene glycol base lasts a long time, but the corrosion inhibitors deplete on schedule. Depleted inhibitors leave old coolant slightly acidic, attacking aluminum engine components and water pump seals from the inside.
- Overfilling the reservoir — coolant expands significantly when the engine reaches operating temperature. Filling to MAX when cold creates overflow when hot. The MIN cold mark is the correct fill target when topping off a cold engine.
Sources
DOE AFDC — Engine Cooling Systems Explained
EPA — Antifreeze & Coolant Environmental Guidance
