Intervals by coolant type
Colour is a convention, not a standard — different manufacturers use the same dye for different chemistries. Go by the specification in your manual or on the bottle, never by colour alone.
| Type | Colour (typical) | Interval |
|---|---|---|
| IAT (conventional) | Green | 2 years / 30,000 mi |
| OAT | Orange, red, pink | 5 years / 150,000 mi |
| HOAT | Yellow, turquoise, blue | 5 years / 150,000 mi |
Why mixing is a real problem
Mixing incompatible chemistries can cause the additive packages to drop out of solution, forming a gel that blocks the radiator, heater core and small coolant passages. It is one of the few maintenance mistakes that can turn into a several-thousand-dollar repair.
If you do not know what is in the system, drain and flush with distilled water before refilling with the correct coolant.
Signs the coolant is due
- Rusty, brown or cloudy fluid in the expansion tank.
- Visible particles or an oily film floating in the tank.
- Heater output that has weakened over time.
- Temperature gauge creeping higher than usual in traffic.
- A test strip showing depleted inhibitors or wrong pH.
Doing it properly
- Only ever open the system when the engine is stone cold.
- Drain the radiator, and the block drain if the engine has one.
- Flush with distilled water until it runs clear. Never use tap water — minerals cause scale.
- Refill with the correct coolant, pre-mixed or diluted 50/50 with distilled water.
- Bleed the system. Air pockets cause hot spots and false gauge readings; many modern engines need a specific bleed procedure or a vacuum fill tool.
- Run to full temperature with the heater on, recheck the level cold, and top up.
Frequently asked questions
- Can I just top up coolant instead of flushing?
- Topping up restores level, not the corrosion inhibitors. If the fluid is old or discoloured, it needs replacing regardless of level.
- Is a chemical flush additive necessary?
- Rarely, for a system in reasonable condition. Distilled water flushing is enough. Chemical flushes are for heavily contaminated systems and can dislodge debris that then blocks the heater core.
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