What Happens If You Mix Different Types of Coolant?
Not all coolant is chemically the same, despite most of it looking similarly coloured, and mixing incompatible types can genuinely cause damage rather than just being a harmless variation.
Why coolant types differ
Different coolant formulations, broadly categorised by the type of corrosion inhibitor technology used, are chemically different from each other, not just different colours for branding. Some use older inorganic additive technology, others use organic acid technology, and some use a hybrid of both. Mixing incompatible types can cause the corrosion inhibitors to react, form deposits, or lose effectiveness, potentially clogging fine passages in the radiator or heater core over time.
Colour is not a reliable guide
It's tempting to assume coolant colour tells you the type, but manufacturers use colour somewhat inconsistently, and two coolants of the same colour aren't necessarily the same formulation. The safe approach is checking the specification, not the colour, ideally against what the handbook specifies for your car.
What to do if you're topping up and unsure
If you don't know what's already in the system, the safest option is a coolant explicitly labelled as compatible with a wide range of types, or getting the system properly drained and refilled with a single, correct type rather than guessing and topping up with something potentially incompatible.
Symptoms of an incompatible mix
A gel-like or discoloured sludge in the coolant, reduced cooling performance, or a coolant warning light appearing more often than expected can all point to a mixing issue, worth having a garage flush and refill the system properly rather than continuing to top up.
Check a car's MOT history for cooling system advisories before you buy.
Check MOT history