Coolant temperature light: the engine is overheating

What the warning actually means

The symbol looks like a thermometer in liquid, or a temperature gauge in the red zone. It means the cooling system is no longer moving heat out of the engine fast enough.

How the cooling system protects the engine

Coolant carries heat from the engine to the radiator, where air takes it away. When that stops working, driving on can warp the cylinder head or blow a head gasket, often within just a few minutes.

That is also why the heater helps. The heater core pulls heat away from the engine and into the cabin, which buys the engine a little margin while you find somewhere to stop.

Common causes

• A coolant leak, often from a radiator damaged by corrosion or road debris
• Blocked airflow through the radiator from debris, bugs or road dirt
• A thermostat stuck closed, so coolant cannot reach the radiator
• A failing water pump
• A cooling fan fault, which shows most at idle or in traffic

What to do now

1. Turn the air conditioning off. The compressor adds load and heat.
2. Turn the heater on full and roll the windows down.
3. Pull over as soon as it is safe and switch the engine off.
4. If you see steam, do not open the hood. Wait for it to clear.
5. Wait at least 30 minutes before checking the coolant level, and only with the engine cold.

What people often get wrong

That the car can be driven on because it seems fine. Engine damage from overheating often is not obvious until it has already happened, and by then it is a repair bill rather than a warning light.

When the car must go to a workshop

Have the system inspected before the car is driven further. White exhaust smoke together with overheating deserves a same-day look. A tow is always cheaper than a new engine.

Read next

• Car losing coolant: where is it going?
• Oil pressure warning light: stop the car now
• Flashing check engine light: slow down, this one is urgent

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.