Why is my car overheating on the highway?

Overheating at highway speed (when there's plenty of airflow through the radiator) usually points to a coolant flow problem rather than a fan problem: stuck thermostat, failing water pump, blockage in the radiator, or a head gasket leak letting combustion gases into the coolant. Highway-only overheating is more serious than city-only — it means even maximum airflow can't keep up.

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