Why is my car overheating?

If your car is overheating, the first step is to stop somewhere safe, turn the engine off and leave it to cool. Do not open the coolant cap while it is hot. Then work out whether the cause is simple, such as low coolant, or something a garage needs to look at.

Most overheating comes from a short list of faults. We take them in turn below, along with what you can safely check.

What should I do if my engine overheats while driving?

Put the heater on full with the fan at maximum, which pulls some heat from the engine, then pull over as soon as it is safe and switch off. Wait until the engine is properly cool before touching anything under the bonnet. The coolant system is pressurised and hot coolant can cause bad scalds.

Do not keep going to get home. A few minutes on an overheating engine can cause damage that is far costlier than waiting for help.

What are the most common causes of overheating?

Low coolant is the first thing to suspect, and it usually means a leak somewhere. A thermostat stuck shut stops coolant reaching the radiator. A radiator blocked inside, or one with a leak, cannot shed heat. A failed cooling fan means the engine overheats in traffic but cools down at speed.

Less common but more serious causes include a failing water pump and a head gasket fault, where combustion gases enter the cooling system.

• Low coolant from a leak
• Thermostat stuck closed
• Cooling fan or fan relay fault
• Blocked or leaking radiator
• Failing water pump
• Head gasket problem

How can I tell which fault it is?

The pattern helps. Overheating only when stopped in queues often points to the fan. Overheating at steady speed on a motorway suggests a flow or radiator problem. A fault that comes and goes might be a thermostat or a failing sensor. A gauge reading that jumps about can be a sensor rather than a real temperature issue.

Signs of a head gasket fault include white smoke from the exhaust, milky oil on the filler cap and coolant that vanishes with no visible leak. These need a garage, not a home fix.

What can I check safely myself?

Only when the engine is fully cold. Look at the coolant level against the marks on the expansion tank, and look under the car for puddles. Check that the fan runs when the engine is warm, and look at the front of the radiator for leaves and debris.

If you top up, use the coolant type in your handbook, because different types should not be mixed. If the level drops again, there is a leak and the car should be seen before it is used for any distance.

What does a garage do about an overheating car?

A garage pressure tests the cooling system to find leaks, tests the thermostat and fan, checks the radiator and looks for signs of combustion gases in the coolant. The fix could be as small as a hose or as large as a head gasket, and we will not know which until we have inspected it.

Cost depends on the part, how hard it is to reach and whether the engine was damaged by running hot. Automaster is at Longcombe Cross near Totnes, and you can ask for a quote once you have told us the symptoms and when they happen.