Coolant temperature light: red, blue, flashing, what to do
Updated · Autofy
Coolant temperature light: red, blue, flashing, what to do
The red thermometer-in-water light means the engine is overheating or the coolant level is too low: stop as soon as it's safe and switch the engine off. Above all, never open the reservoir while the engine is hot: pressurised coolant can cause serious burns. Wait at least thirty minutes, check the level and top up if needed. A blue light, on some cars, simply means the engine is still cold.
Red, blue, flashing
- Red: overheating, or level too low. Stop immediately.
- Flashing red: on many cars, low coolant flagged before overheating. Treat it like red.
- Blue: engine still cold, on some makes. Normal for the first few minutes.
- Temperature needle in the red, with no light: same advice as the red light.
What to do, step by step
- Pull over as soon as it's safe and switch the engine off.
- Don't open the bonnet straight away if steam is coming out.
- Never open the reservoir when hot. The system is pressurised: coolant can spray out at close to 100 °C.
- Wait at least thirty minutes, then check the level in the expansion tank: it should sit between min and max with a cold engine.
- Level low? Top up with the recommended coolant, or water in an emergency, and look for the leak.
- Level fine but the light comes back? Thermostat, water pump or cooling fan: garage, driving gently.
What makes an engine overheat
- A leak: split hose, loose clip, holed radiator, weeping water pump.
- The thermostat stuck closed, stopping coolant from reaching the radiator.
- The cooling fan no longer kicking in, a classic in traffic jams.
- A worn water pump. On many engines it's driven by the timing belt and replaced with it: see our timing belt guide.
- The head gasket, when coolant disappears with no visible leak, with white smoke or a creamy deposit under the oil cap.
If the light comes on together with the battery light, think of the accessory belt: on many engines it also drives the water pump.
And the coolant itself
Coolant ages: its anti-corrosion additives wear out. Replacement intervals vary widely between carmakers and coolant types, from a few years to "long-life" fluids. The only figure that counts is the one in your service book.
Keep track of what's been done
Autofy keeps the history of every service, with a photo of the invoice, so you can see at a glance when the water pump or coolant was last changed. Every other light is in our dashboard warning lights guide.