Car overheating: what to do and what causes it
An overheating engine is one of the most serious situations you can face as a driver. This guide explains exactly what to do right now, the most common causes, and what it costs to fix each one.
Temperature in the red zone right now?
Pull over safely, turn off the engine, and do not open the bonnet until the engine has cooled for at least 15 minutes. Do not restart or continue driving. Call your breakdown provider or get a mobile mechanic to come to you.
What to do right now
Pull over safely and stop the engine
The moment you notice the temperature gauge entering the red zone, look for a safe place to pull over. Do not continue driving. Turn off the engine as soon as you have stopped safely. Turning off the air conditioning first and switching the heater to maximum heat and maximum fan may buy you a few minutes of cooling time if you need to reach a safer stopping point.
Wait before opening the bonnet
After stopping, wait at least 15 to 20 minutes before opening the bonnet. The cooling system is pressurised and extremely hot. Opening the bonnet immediately is safe; opening the radiator cap or coolant reservoir cap while the system is pressurised risks a severe steam and boiling coolant burn. Leave the bonnet closed until you can touch the area around the radiator without pain.
Check the coolant level (safely)
Once the engine has cooled sufficiently, open the bonnet and locate the coolant reservoir (a translucent plastic tank, usually near the radiator, with MIN and MAX markings). Check the level. If it is below the MIN mark or empty, a coolant leak is likely. Do NOT open the radiator cap while the engine is still warm. If the reservoir is empty, do not restart the engine until the cause of the coolant loss is known.
Do not restart if you are unsure
If the coolant level is critically low, the engine has white smoke, or you are unsure of the cause, do not restart the engine. Call your breakdown provider, or arrange for the car to be recovered to a garage. Driving an overheating car risks engine seizure, which can turn a £500 repair into a £3,000 to £5,000 engine replacement.
If coolant is present and engine has cooled, proceed with caution
If the coolant level is normal, the engine cooled down and restarted normally, and the temperature gauge returns to normal within a few minutes of driving, the issue may be intermittent (cooling fan, thermostat). Drive directly to a garage with the heater on full heat and monitor the temperature gauge continuously. Stop immediately if it rises again.
Common causes of car overheating
Coolant leak
Most common causeThe engine cooling system is a closed circuit. If coolant leaks out, the remaining coolant cannot absorb heat fast enough and the engine overheats. Leaks can be external and visible (puddles under the car, dripping from hoses or the radiator) or internal (coolant leaking into the engine or exhaust system without any external puddle). External leaks are caused by failed hoses, a cracked or leaking radiator, a failing water pump seal, or loose hose clamps. Internal leaks are typically caused by a blown head gasket or, in severe cases, a cracked cylinder head.
Symptoms:
Low coolant warning light, visible coolant puddles under the car, sweet smell from the engine bay, white steam (not smoke) from the bonnet, temperature gauge rising.
Typical repair cost: Hose replacement: £80 to £200. Radiator replacement: £300 to £600. Water pump: £250 to £500. Head gasket: £700 to £1,500.
Urgency: Stop driving immediately if the temperature gauge is in the red. Continuing to drive with a serious coolant leak can seize the engine within minutes.
Failed or stuck thermostat
Very commonThe thermostat is a small valve that regulates coolant flow through the radiator. When the engine is cold, it stays closed to let the engine warm up quickly. Once the engine reaches operating temperature, it opens to allow hot coolant to flow through the radiator and cool down. A thermostat stuck closed prevents coolant from reaching the radiator, causing the engine to overheat rapidly. A thermostat stuck open causes the engine to run too cool (poor fuel economy, slow warming), but not overheating.
Symptoms:
Temperature gauge rising rapidly (thermostat stuck closed), engine overheating with no coolant loss, engine running unusually cold (thermostat stuck open).
Typical repair cost: Thermostat replacement: £100 to £250 including parts and labour.
Urgency: A stuck-closed thermostat will cause overheating very quickly. Stop driving and arrange repair rather than topping up coolant and continuing.
Failed water pump
CommonThe water pump circulates coolant around the engine and through the radiator. It is driven by the cambelt (timing belt) or a separate drive belt. Water pump failure stops coolant circulation, leading to rapid overheating. The pump can fail due to bearing wear (accompanied by a rumbling or whining noise), impeller damage, or seal failure (causing a leak around the pump body). On cambelt-driven pumps, the water pump is typically replaced at the same time as the cambelt to save on labour costs.
Symptoms:
Overheating with no obvious external leak, whining or grinding noise from the front of the engine, coolant leak from the pump body (front of the engine block), temperature gauge rising quickly.
Typical repair cost: Water pump replacement: £200 to £500 depending on the car. If replaced alongside the cambelt: add £100 to £200 to the cambelt job cost.
Urgency: If the water pump has seized, driving can snap the cambelt (on belt-driven pumps), causing catastrophic engine damage. Stop immediately.
Radiator blocked or damaged
Common on older carsThe radiator dissipates heat from the coolant using airflow through its fins. Old coolant that has not been changed on schedule becomes acidic and deposits scale inside the radiator, reducing its efficiency. Physical damage to the radiator fins (from stone chips or front-end impacts) also reduces airflow. A partially blocked radiator may only overheat in slow traffic where airflow is low, working fine at motorway speeds.
Symptoms:
Overheating in slow traffic but normal temperature at higher speeds, visible physical damage to the front of the radiator, coolant that looks rusty or discoloured.
Typical repair cost: Radiator flush: £80 to £150. Radiator replacement: £300 to £600 including parts and labour.
Urgency: A blocked radiator tends to cause gradual overheating rather than sudden temperature spikes. Monitor the gauge carefully in traffic.
Cooling fan failure
Common on electric-fan carsMost modern cars use an electric cooling fan that switches on when the engine reaches a certain temperature or when the air conditioning is running. If the fan motor burns out, the relay fails, or a sensor does not trigger it, the radiator will not receive forced airflow at low speed. At motorway speeds, the car's forward motion provides enough airflow and the temperature stays normal. But in traffic, without the fan, the engine can overheat.
Symptoms:
Engine overheats in slow traffic or when stationary but runs normally at higher speeds, no audible fan noise from the engine bay when the car is idling hot.
Typical repair cost: Cooling fan motor replacement: £150 to £350. Fan relay: £20 to £80. Temperature sensor: £80 to £180.
Urgency: Turn on the car's heater to maximum heat and maximum fan speed. This draws heat away from the engine and can buy time to reach a garage safely. Do not ignore rising temperatures in traffic.
Blown head gasket
Serious but less commonThe head gasket seals the cylinder head to the engine block, keeping combustion gases, coolant, and oil in their separate circuits. A blown head gasket can allow combustion gases to enter the cooling system (pushing coolant out of the reservoir), coolant to enter the cylinders (white exhaust smoke), or oil and coolant to mix. Head gaskets typically fail due to overheating from another cause (such as a coolant leak or failed thermostat), creating a cycle where overheating causes the gasket to fail, which then causes further overheating.
Symptoms:
White sweet-smelling smoke from the exhaust, milky creamy residue on the oil filler cap or dipstick, coolant level dropping with no external leak, bubbling in the coolant reservoir, overheating despite normal coolant level.
Typical repair cost: Head gasket replacement: £700 to £1,500 for most cars. If the head is warped and needs skimming: add £200 to £400.
Urgency: A blown head gasket requires immediate repair. Driving with a confirmed head gasket failure will cause the engine to seize or the cylinder head to warp, multiplying repair costs significantly.
Need a mechanic to come to you?
If your car has overheated and you are stuck, a mobile mechanic can come to your location and diagnose the problem on the spot. Post your job on AutoFixFair to compare quotes.
Frequently asked questions
What are the first signs of a car overheating?
The first sign is usually the temperature gauge rising toward or into the red zone on the dashboard. Some cars also have a separate temperature warning light (a thermometer symbol in red). You may also notice a sweet smell from the engine bay (coolant), steam coming from under the bonnet, or the heater suddenly blowing cold air (indicating no coolant circulation). Never ignore a rising temperature gauge.
Can I drive my car if it is overheating?
No. You should stop the car as soon as it is safe to do so when the temperature gauge enters the red zone. Continuing to drive an overheating engine can cause catastrophic and irreversible damage within minutes, including a seized engine requiring a full replacement. The cost of stopping and getting the car recovered is trivial compared to the cost of an engine replacement.
What happens if you ignore an overheating engine?
In the short term, the cylinder head can warp or crack. The head gasket can blow. In the most extreme cases, the engine can seize completely, requiring full replacement. Any of these outcomes turns what might have been a £100 to £500 repair (a thermostat, a hose, a water pump) into a £2,000 to £6,000 repair or write-off. Stopping immediately when the gauge rises is the single most important thing you can do.
Why is my car overheating in traffic but not on the motorway?
This pattern strongly suggests a cooling fan fault. At motorway speeds, airflow through the radiator keeps the engine cool without the fan. In slow traffic or when stationary, the car relies on the electric cooling fan. If the fan motor, relay, or sensor has failed, the radiator receives no forced airflow and the engine gradually overheats. Turn on the heater to maximum as a temporary measure and get the car checked immediately.
My coolant level is fine but the car is overheating. What is wrong?
If the coolant level is correct but the engine overheats, the most likely causes are a stuck thermostat (coolant is not circulating to the radiator), a failed cooling fan (no airflow through the radiator at low speed), a failed water pump (coolant is not being circulated), or a partially blocked radiator. A garage can diagnose the specific cause with a pressure test and visual inspection.
How much does it cost to fix an overheating car?
The cost depends entirely on the cause. A thermostat costs £100 to £250 to replace. A cooling fan motor costs £150 to £350. A radiator costs £300 to £600. A water pump costs £200 to £500. A head gasket costs £700 to £1,500. Getting the cause diagnosed first (£50 to £90 for a diagnostic check) is the most cost-effective approach before agreeing to any parts replacement.
Can adding coolant fix an overheating problem?
Topping up coolant can resolve the immediate symptom if the overheating was caused by a low coolant level due to a minor leak. However, it does not fix the underlying cause. If the coolant level drops again, the leak needs to be found and repaired. Never pour cold water into a hot engine or open the coolant reservoir cap while the engine is still hot.