Engine temperature warning light

The engine temperature light is a thermometer, often standing in wavy lines that represent coolant. When it lights up red, or the temperature needle swings into the red, the engine is too hot. This is a stop now warning: an overheating engine can warp metal parts, damage the head gasket and, at worst, wreck the engine. AAA's advice is plain: continuing to drive can damage or destroy the engine.

Some cars also have a blue thermometer light. That one only means the engine is still cold and is not a fault.

What to do right now

1. Pull over as soon as it is safe and switch off the engine.
2. Do not open the bonnet (hood) straight away if you see steam. Let it settle first and keep your face and hands away.
3. Never open the radiator cap or the coolant expansion tank while the engine is hot. The system is pressurised and can spray boiling coolant and steam, causing severe burns.
4. Wait at least 30 minutes for the engine to cool. The RAC says an hour or two is safer.
5. Once it is cold, check the coolant level in the expansion tank against the min and max marks. Open the cap slowly with a cloth over it. Top up with the coolant your owner's manual specifies if it is low.
6. Look under the car for leaks, such as drips or a puddle of coloured fluid near the front.
7. Restart and watch the gauge. If the light comes back, do not drive on. Call your breakdown service.

Likely causes, most common first

1. Low coolant or a leak

   A common cause. Coolant can leak from hoses, the radiator or the water pump, and older engines can slowly use it. The RAC notes the temperature light can be your first sign of low coolant.

   Read the fix guide →

2. Stuck thermostat

   The thermostat opens to let coolant flow to the radiator. If it sticks closed, the engine heats up quickly, even with plenty of coolant.

3. Failed cooling fan

   The fan pulls air through the radiator when you are moving slowly or standing still. If it stops working, the engine overheats in traffic but may be fine at speed.

4. Failing water pump

   The pump circulates coolant. A worn pump may leak, whine or simply stop moving enough coolant.

5. Head gasket failure

   Often the result of overheating rather than the first cause. Signs include white exhaust smoke, coolant disappearing with no visible leak, or milky, discoloured engine oil. A garage can test for it.

   Read the fix guide →

Hot weather and heavy loads, such as towing or climbing long hills, make all of these worse.

What it costs to fix

Topping up coolant or replacing a split hose is usually a small bill. Bigger jobs depend a lot on the car. As one example of a common repair:

DIY or garage?

How to prevent it

• Check the coolant level in the expansion tank every few weeks, with the engine cold.
• Use the coolant type your owner's manual specifies, not plain water.
• Have the cooling system flushed at the interval in your owner's manual.
• Get small leaks or a rising temperature gauge checked early.
• Never ignore the gauge creeping higher than usual, especially in summer traffic.