Overheating: what to do right there on the road

The needle is climbing. The next five minutes decide whether this costs a thermostat or a cylinder head.

Overheating is one of the few problems where minutes matter and the right actions cost nothing.

Do this at once

1. Switch the air conditioning off and the heater to maximum. The heater is a second radiator: it takes away part of the heat. The cabin becomes unbearable; put up with it.
2. Do not switch the engine off straight away if the needle is not yet in the red. At idle the pump keeps the coolant moving, while a hot engine switched off cools unevenly. Pull over and let it idle for a minute or two.
3. If the needle is in the red or there is steam from under the bonnet, stop the engine immediately. Subtleties no longer apply.
4. Do not open the expansion tank cap. The system is under pressure and boiling coolant sprays upwards. Wait at least half an hour.

What to look at once it has cooled

• The coolant level. Empty means finding where it goes: a puddle under the car, white smoke from the exhaust, emulsion under the oil filler cap.
• The temperature as a number, not as a needle. On many cars the needle deliberately sticks in the middle of the scale and shows trouble last.
• Whether the fan comes on. With the car standing and warm it must start by itself. If it does not, the relay or the fuse is the cheapest of the causes.

Common causes in order

1. Low coolant or air in the system.
2. The fan does not run — the relay or the fuse.
3. A thermostat stuck closed. The opposite case, stuck open, gives P0128 and a cold heater rather than overheating.
4. The water pump or a blocked radiator.
5. The head gasket — more often a consequence of prolonged overheating than a cause.

What not to do

Drive «just a bit, to get home». Every kilometre on an overheated engine trades a cheap repair for an expensive one.

Switch off and start again straight away. If the coolant is low, the next start heats it even faster.