Why Is My Car Overheating?
Low coolant is the usual culprit, but thermostats, water pumps and radiators fail too. What to do straight away and why you shouldn’t drive on.
A car usually overheats because of low coolant, often from a leak, or because something stops coolant circulating: a stuck thermostat, a failed water pump, a blocked radiator or a faulty cooling fan. Whatever the cause, stop as soon as it’s safe. An engine driven while overheating can suffer serious damage in a short time.
What to do if the gauge climbs or steam appears
- Stop somewhere safe. Pull over away from moving traffic as soon as you can. If you can’t stop right away, turn off the air conditioning and turn the heater to full, because that draws some heat out of the engine. It will be uncomfortable, but it helps.
- Switch off the engine. Pop the bonnet catch from inside, but don’t lift the bonnet yet if there’s a lot of steam coming out.
- Get everyone clear. Stay well away from traffic. On a motorway, wait behind the barrier if it’s safe to get there.
- Wait. Give the engine at least 30 minutes to cool before opening the bonnet or touching anything.
- Check the level. When it’s cold, look at the coolant bottle. Its min and max marks are on the side.
Never open a radiator or expansion tank cap on a hot engine. The system is under pressure, and it can spray boiling coolant and steam over you. Scalds from this are a real risk. Also don’t pour cold water into a hot engine, as it can cause damage.
Coolant is toxic and tastes sweet, so keep it away from children and pets and clean up any spills.
Common causes
Low coolant
This is the most common cause, and coolant doesn’t disappear on its own. If the level has dropped, there’s a leak somewhere: a hose, a clip, the radiator, the water pump or an internal engine leak. Look under the car for puddles, often green, pink, blue or orange, and for crusty residue around joints.
Stuck thermostat
The thermostat opens to let hot coolant through to the radiator. If it sticks shut, the engine can heat up fast, sometimes within minutes of setting off, while the radiator stays cool. See our thermostat replacement page for more.
Failed water pump
The pump pushes coolant round the engine. Warning signs can include coolant leaks around the front of the engine and a whine or rumble that rises with revs. On many cars the pump is driven by the cam belt, so a water pump replacement and a cam belt change are often sensibly done together.
Blocked or leaking radiator
Internal scale, debris in the fins or even a plastic bag caught on the grille can stop the radiator shedding heat. Corrosion can also cause leaks along the seams. Our radiator repairs page explains the options.
Cooling fan faults
If the car is fine on the move but the temperature climbs in traffic, that can point to the electric cooling fan, its fuse or its sensor. At speed, moving air does the job, so the fault only shows up when you’re sitting still.
Head gasket failure
Coolant loss with no visible leak, white smoke from the exhaust, bubbles in the coolant bottle, or a milky deposit on the oil cap can suggest head gasket trouble. Those signs aren’t proof, since a milky cap is often just condensation on short-trip cars, but they need proper testing. This is usually a workshop repair.
Can I top up and carry on?
If the engine has fully cooled, the level is low and you’re near your destination, a careful top-up may be enough to get you moved to a safe place. Use the coolant type your handbook lists; plain water is an emergency measure only, because it dilutes the antifreeze protection. Watch the gauge the entire way, and stop again if it rises. If the car lost coolant quickly, or there’s steam or smoke, don’t drive it. Arrange recovery or have it looked at where it is.
Prevention
Check the coolant level regularly with the engine cold, as your handbook advises, and have a low level looked into rather than simply topping up every week. Have hoses checked at service time, and get any sweet smell from the heater vents or damp carpet in the passenger footwell investigated, as that can point to a heater matrix leak.
If your car has overheated, tell us the make and model, what the gauge did, whether you lost coolant, and where it’s parked. Many cooling system repairs can be carried out on site, subject to the suitability of the job and location.