Why is my car overheating?

This part matters more than the diagnosis. If the temperature gauge is heading into the red, turn off the air conditioning and turn the heater on full. The heater core acts as a second radiator and pulls real heat out of the engine, however unpleasant that is in July.

Then pull over safely and shut the engine off. Do not open the radiator cap on a hot engine — the system is pressurized and the coolant is well above boiling point. Let it cool, and call.

Overheating means the cooling system has lost either coolant or the ability to move heat. A handful of failures account for most cases.

Overheating in stop-and-go traffic but staying normal on the highway is a strong indicator of a cooling fan problem, because at speed the airflow happens without them.

Overheating at highway speed but staying normal in town points more toward coolant flow: a thermostat, water pump or a restricted radiator that cannot keep up with the higher heat load.

Heat is what destroys engines. Aluminum cylinder heads warp, head gaskets fail, and severe overheating can damage pistons and bearings. A single bad overheat can cause damage that was not there the day before.

That is why a cooling-system repair is one of the cheapest things you can do relative to what it prevents. A hose or thermostat is a small job. A head gasket is not, and a replacement engine is in another category entirely.