On the spot
If the temperature gauge climbs into the red or a coolant warning comes on, pull over safely and switch the engine off. Do not open the radiator or coolant cap while hot; it is pressurised and can scald. Let it cool fully before checking the coolant level.
Common causes
Low coolant from a leak, a failed thermostat, a faulty radiator fan, a failed water pump, or a blocked radiator. A sweet smell, steam, or puddles of green, pink or orange fluid under the car point to a leak.
Head gasket worries
Repeated overheating can damage the head gasket. White exhaust smoke with a sweet smell, coolant loss with no visible leak, or oil that looks milky are signs to mention.
What to tell the mechanic
When it overheats (in traffic, on the motorway, uphill), whether the heater blows cold, any leaks you have seen, and how far it was driven hot.
Safe to drive?
Do not keep driving an overheating engine. Topping up coolant once it has cooled may get you a short distance, but if it overheats again, have it towed.
General guidance only, not a diagnosis. If in doubt, do not drive it and have it inspected. In an emergency, call 111.
Who can help
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Questions
Can I just add water?
In an emergency, water gets you moving, but the cooling system should be refilled with the correct coolant mix and the leak found.
Why does it only overheat in traffic?
That often points to the radiator fan not switching on, since there is no airflow from driving.