The red thermometer light means the engine is too hot. Safe steps to take, why you must not open a hot radiator cap and common causes.
Looking for more detail? Our full guide covers this topic in depth, with step-by-step advice and sources.
Coolant temperature is too high, so the engine is at risk of overheating.
What it looks like
A thermometer in wavy lines (liquid). Colour: Red (some cars show blue when the engine is cold).
Common causes
- Low coolant from a leak
- Failed thermostat or water pump
- Cooling fan not working
- Blocked radiator
What to do
- Pull over safely and switch off the engine.
- Do not open the coolant cap while hot: pressurised coolant can scald.
- When cool, check the coolant level against the marks on the expansion tank and top up with the correct coolant if needed. If it keeps happening, get it inspected.
Can I keep driving?
No. Driving an overheating engine can warp the cylinder head.
MOT
Not a direct MOT item, but coolant leaks may be noted.
Symbols vary slightly between makes. Always check your owner's handbook. General guidance only; if in doubt, call your breakdown provider.
Sources and further reading
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.