A car overheating is something you should never ignore. Even if the vehicle still appears to be driving normally, continuing your journey with an excessively hot engine can cause serious and potentially expensive damage.
Sometimes, the cause is relatively straightforward, such as a low coolant level. In other cases, overheating can point to a problem with the radiator, cooling fan, thermostat, water pump or even the engine itself.
In this guide, we explain the most common reasons why a car overheats, the warning signs to look out for and what to do if the temperature begins to rise.
Most modern cars have either an engine-temperature gauge or a dashboard warning light.
Under normal driving conditions, the temperature gauge should remain around the middle of its range once the engine has warmed up. If the needle moves towards the red section, or a red temperature warning light appears, the engine may be overheating.
Other signs can include:
You may notice several of these signs together, although an overheating problem can sometimes develop without any visible steam.
Your engine produces a significant amount of heat while it is running. The cooling system circulates coolant around the engine and transfers this heat through the radiator.
If coolant is unable to circulate properly, or the cooling system cannot release enough heat, the engine temperature will begin to rise.
Below are some of the most common causes.
1. Low coolant level
A low coolant level is one of the most common reasons for a car to overheat.
Coolant absorbs heat from the engine and carries it towards the radiator, where that heat is released. If there is not enough coolant in the system, the engine may become too hot.
Coolant levels can fall because of:
Coolant should not normally disappear quickly. If you regularly need to top it up, there is likely to be a leak or another fault that needs investigating.
Never remove the coolant cap while the engine is hot. The cooling system may be pressurised, and hot coolant can cause serious burns.
2. Coolant leak
A coolant leak can happen almost anywhere within the cooling system.
You may notice a coloured liquid underneath the car, although the exact colour depends on the type of coolant being used. Coolant is commonly pink, red, blue, green, orange or yellow.
Some leaks only appear when the engine is hot and the system is under pressure, so there may not always be an obvious puddle beneath the vehicle.
A garage may carry out a cooling-system pressure test to help locate the source of the leak.
3. Faulty thermostat
The thermostat controls the flow of coolant between the engine and the radiator.
When the engine is cold, the thermostat remains closed to help the engine warm up efficiently. As the temperature increases, it opens and allows coolant to flow through the radiator.
If the thermostat becomes stuck closed, the coolant cannot circulate properly and the engine may quickly overheat.
A stuck thermostat may cause the temperature gauge to rise suddenly, even during a relatively short journey.
4. Radiator problem
The radiator removes heat from the coolant before it circulates back through the engine.
Overheating may occur if the radiator is:
A blocked radiator may prevent coolant from circulating effectively, while damaged radiator fins can reduce the amount of heat being released.
5. Cooling fan not working
The radiator fan helps pull air through the radiator, particularly when the vehicle is stationary or travelling slowly.
If the cooling fan fails, the car may remain at a normal temperature while driving at speed but begin overheating in traffic, while idling or when parked with the engine running.
Cooling-fan problems can be caused by:
If your car mainly overheats in slow-moving traffic but cools down once you begin driving, the radiator fan is one possible cause.
6. Faulty water pump
The water pump circulates coolant around the engine and cooling system.
If the pump fails or becomes damaged, coolant flow can be reduced or stop completely. This can lead to rapid overheating.
Possible signs of a faulty water pump include:
On some vehicles, the water pump is driven by the timing belt. This means replacement may involve significant labour, and it can be sensible to replace related timing-belt components at the same time.
7. Damaged or collapsed coolant hose
Coolant travels between the engine and radiator through a series of hoses.
A hose can split, leak, soften or collapse internally. This may restrict coolant flow even when there is no major external leak.
Older hoses can become brittle over time, while oil contamination or excessive pressure can also cause deterioration.
8. Air trapped in the cooling system
Air pockets can prevent coolant from circulating correctly.
This is sometimes known as an airlock and can happen after coolant has been drained, a hose has been replaced or work has been carried out on the cooling system.
Signs of trapped air may include:
The cooling system may need to be correctly bled to remove trapped air.
9. Faulty temperature sensor
The engine coolant-temperature sensor monitors how hot the engine is.
A faulty sensor may send incorrect information to the dashboard or engine-control unit. It could also prevent the cooling fan from switching on at the correct time.
In some cases, the engine may not actually be overheating, but the dashboard may display an inaccurate temperature reading. However, you should always treat a high-temperature warning seriously until the cause has been confirmed.
10. Head gasket failure
A failed head gasket is one of the more serious potential causes of overheating.
The head gasket creates a seal between the engine block and cylinder head. If it fails, coolant and engine oil may mix, combustion gases may enter the cooling system or coolant may leak into the cylinders.
Possible signs include:
These symptoms do not always confirm head-gasket failure, so proper diagnosis is essential.
11. Low engine-oil level
Engine oil helps lubricate moving components and also contributes to controlling engine temperature.
If the oil level is extremely low, friction and heat can increase. This may contribute to overheating and can cause severe internal engine damage.
If both the oil warning light and temperature warning light appear, stop driving as soon as it is safe to do so.
Regular maintenance can help identify low fluid levels and developing mechanical problems before they cause a breakdown. Read our guide to car service costs in the UK and what a service includes for more information.
A car that overheats when stationary but cools down while moving may have an airflow problem.
When you are driving, air naturally passes through the radiator. When the car is stationary, the cooling fan must provide this airflow.
Possible causes include:
Traffic and warm weather can make the problem more noticeable.
If your car overheats while travelling at higher speeds, the problem may relate more closely to coolant circulation than radiator airflow.
Possible causes include:
Continuing to drive at speed while the engine is overheating can rapidly worsen the damage.
A full coolant reservoir does not necessarily mean the cooling system is working properly.
The vehicle could still have:
It is also possible for the expansion tank to appear full while the radiator or another part of the system contains insufficient coolant.
If the engine is overheating but the cabin heater is blowing cold air, coolant may not be circulating through the heater matrix correctly.
This can be caused by:
This combination of symptoms should be checked promptly, as it can indicate that coolant circulation has been severely restricted.
If the temperature warning light comes on or the gauge enters the red zone, take the following steps.
1. Turn off the air conditioning
Air conditioning places additional load on the engine. Switching it off may slightly reduce the amount of heat being generated.
However, an overheating engine and an air-conditioning system that is not cooling properly are usually separate issues. If the cabin is no longer getting cold but the engine temperature remains normal, read our guide to air-conditioning recharge costs and signs your air con needs attention.
2. Turn the heater on
Turning the heater to its highest temperature can help draw a small amount of heat away from the engine.
This is only a temporary measure to help you reach a safe stopping place. It does not repair the underlying fault.
3. Pull over safely
Stop somewhere safe as soon as possible. Turn the engine off and allow it to cool.
Do not continue driving in the hope that the problem will resolve itself.
4. Do not open the coolant cap
Never remove the radiator or coolant-reservoir cap while the engine is hot.
The system may contain pressurised boiling coolant, which can escape suddenly and cause serious injury.
5. Wait for the engine to cool
Allow at least 30 minutes for the engine to cool before inspecting anything beneath the bonnet. A badly overheated engine may take considerably longer.
6. Check for obvious leaks
Once the engine has cooled, look underneath the vehicle and around the engine bay for signs of leaking coolant.
Do not place your hands near hot or moving engine components.
7. Check the coolant level when safe
Only check the coolant once the engine is completely cool.
If the level is low, topping it up may help temporarily, but the cause of the coolant loss still needs to be investigated.
Use the coolant specification recommended by the vehicle manufacturer. Mixing incompatible coolant types can cause problems within the cooling system.
8. Arrange recovery or professional inspection
If the temperature rose into the red zone, steam was visible or the coolant level has dropped significantly, arranging recovery is usually safer than driving the vehicle.
A garage can inspect the cooling system, test for leaks and identify the underlying cause.
You should not continue driving a car that is overheating.
Even a short journey can cause:
The cost of recovery is likely to be far lower than the cost of replacing or rebuilding an engine.
In an emergency, plain water may be used as a temporary measure once the engine is completely cool.
However, water does not offer the same corrosion, lubrication, boiling-point or freezing protection as the correct coolant mixture.
The cooling system should be inspected and refilled with the correct coolant as soon as possible.
Never pour cold water into a very hot engine, as the sudden temperature change may cause damage.
The cost depends entirely on the cause.
A simple replacement hose or minor coolant leak may be relatively inexpensive to repair. A thermostat, radiator, cooling fan or water pump will usually cost more because of the parts and labour involved.
A damaged head gasket or engine can be significantly more expensive to repair.
Getting the problem checked quickly can prevent a smaller cooling-system fault from developing into major engine damage.
An overheating engine can quickly develop into a much more serious problem, so it is best to act early.
Whether the cause is low coolant, a leaking hose, a faulty thermostat or a radiator problem, a local garage can inspect the cooling system and recommend the correct repair.
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