Temperature warning light on: causes and what to do

The most frequent cause of an overheating warning is a coolant level that has dropped below the minimum threshold in the expansion tank. Engine coolant (also called antifreeze) circulates through the engine block and cylinder head to absorb heat, then passes through the radiator to shed that heat into the airflow. If the coolant level is too low, the pump circulates less fluid and heat transfer from the engine metal to the coolant deteriorates rapidly. Even a partial loss of coolant can push temperatures to dangerous levels within minutes of running the engine, particularly in slow traffic where airflow across the radiator is minimal. On many modern cars a low coolant level indicator appears separately before the temperature warning, but not all vehicles have this feature.

Typical repair cost: Coolant top-up: £10 to £20 for the fluid. If low coolant has caused no lasting damage, no further repair is needed. If the low level was caused by a leak: repair costs vary from £60 (hose clip) to £800 (coolant leak at the cylinder head).

A coolant leak allows fluid to escape the closed cooling system, progressively reducing the volume available to absorb engine heat until the temperature warning illuminates. Leaks can occur at multiple points: coolant hoses (which degrade and split with age), the radiator (which can corrode or crack), the water pump housing, the expansion tank, heater matrix connections, or at the cylinder head gasket (which is the most serious type of leak). External coolant leaks are usually visible as a sweet-smelling puddle of bright green, pink, or orange fluid under the car, or as a white deposit around hose connections. Internal leaks into the combustion chamber or engine oil are harder to detect and are more damaging.

Typical repair cost: Coolant hose replacement: £60 to £200. Radiator replacement: £200 to £500. Water pump replacement: £200 to £500. Head gasket repair: £600 to £1,500 depending on the engine.

The thermostat is a temperature-controlled valve that sits between the engine and the radiator. When the engine is cold, the thermostat stays closed so coolant circulates only within the engine block and warms up quickly. Once the engine reaches normal operating temperature (typically 85 to 95 degrees Celsius), the thermostat opens and allows hot coolant to flow to the radiator to be cooled. If the thermostat sticks in the closed position, coolant cannot reach the radiator at all, and the engine temperature rises rapidly to dangerous levels. Thermostats are a wear item that become less reliable as vehicles age, and failure in the closed position is the more dangerous mode of failure.

Typical repair cost: Thermostat replacement: £80 to £250 including parts and labour on most vehicles. Thermostats are inexpensive components but access varies considerably between engine designs. Some require significant disassembly.

The water pump is the heart of the cooling system, circulating coolant under pressure from the engine to the radiator and back. On most petrol engines it is driven by the timing belt or a separate auxiliary belt; on some engines it is chain-driven. If the water pump fails, coolant stops circulating and the engine overheats rapidly, even if the coolant level is correct, the thermostat opens correctly, and the radiator is in perfect condition. Water pumps fail in several ways: the impeller (the internal paddle that drives fluid) can erode or shear off the shaft, the shaft bearings can fail (often announced by a rumbling or squealing from the front of the engine), or the seal can leak coolant externally. On timing belt-driven water pumps, pump failure is also a risk of a snapped timing belt, so the two are usually replaced together at service intervals.

Typical repair cost: Water pump replacement (auxiliary belt driven): £150 to £350. Water pump replacement (timing belt driven, including belt replacement): £350 to £700. Chain-driven water pump: £300 to £600.

The radiator dissipates heat from the coolant into the passing air, either through natural airflow when driving or via the electric cooling fan when stationary or in slow traffic. A radiator can fail in two ways: physically (corrosion, impact damage, or internal scaling from using poor quality water causing blockage of the fine internal tubes) or functionally (the cooling fan failing to operate, so the radiator cannot shed heat in traffic). External radiator damage is sometimes visible after a frontal impact, even a minor one such as a low-speed car park collision. Internal scaling is more common on high-mileage vehicles where the coolant was not changed at the recommended interval, allowing the inhibitors that prevent corrosion to degrade.

Typical repair cost: Cooling fan motor or relay replacement: £80 to £250. Radiator flush (to clear internal scaling): £60 to £120. Radiator replacement: £200 to £500 for most vehicles, higher on premium or large vehicles.

The cylinder head gasket seals the joint between the engine block and the cylinder head, preventing coolant, oil, and combustion gases from crossing into one another's passages. When the head gasket fails, it can allow hot combustion gases to pressurize the coolant system (causing rapid overheating and steam from the expansion tank), allow coolant to enter the combustion chamber (producing white smoke from the exhaust), or allow coolant and oil to mix (creating a milky, mayonnaise-like contamination visible on the oil filler cap). Head gasket failure is often caused by a previous overheating event and can itself cause further overheating, creating a destructive cycle. Continuing to drive with a blown head gasket risks warping or cracking the aluminium cylinder head, turning a £600 repair into a £2,000 one.

Typical repair cost: Head gasket replacement: £600 to £1,500 on most four-cylinder engines. If the cylinder head has warped and needs skimming: add £150 to £400. If the head is cracked: cylinder head replacement costs £400 to £1,200 extra.