What Is a Thermostat?
The thermostat is a wax-element valve located in the coolant circuit, typically at the engine outlet to the radiator. When the engine is cold, the thermostat remains closed, restricting coolant flow to the radiator and allowing the engine to warm up quickly. As coolant temperature rises to the thermostat opening temperature — typically 80 to 95 degrees Celsius — the wax element expands, opening the valve and allowing full coolant circulation through the radiator.
What Does the Thermostat Do?
Modern engines must reach their design operating temperature quickly and maintain it precisely for optimal fuel economy, performance and emissions. An engine running below design temperature uses more fuel, produces more emissions and wears more rapidly due to condensation of combustion byproducts in the oil. A thermostat stuck open prevents the engine reaching operating temperature; one stuck closed causes rapid overheating.
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A thermostat stuck open causes the engine to take a very long time to reach operating temperature, poor fuel economy, an ineffective heater and in turbocharged engines possible overboost due to cold-running enrichment. A thermostat stuck closed causes rapid overheating. Both conditions will typically trigger warning lights or temperature gauge deviation from the normal position.
When Should the Thermostat Be Replaced?
Thermostats are inexpensive components (typically £15 to £50) with significant labour to access on some engines. Replace at the first sign of temperature regulation issues or when the cooling system is otherwise opened for service. Always use a genuine or OEM-quality thermostat — cheap pattern parts often have lower opening temperatures that impair performance.
If you are experiencing symptoms related to your thermostat and are unsure whether it needs replacing, a free MotorLoom fault scan can help identify whether the component is likely at fault - giving you the most likely cause and estimated repair cost before you visit a garage.
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Related Parts
Other components that work alongside or are related to the thermostat: