Engine Thermostat Guide
Last updated: 24 April 2026
The engine thermostat is a small but vital cooling system component. It regulates coolant flow between the engine and radiator to maintain optimal operating temperature — typically 85-105 °C. A stuck thermostat causes either overheating (stuck closed) or poor cabin heating and increased fuel consumption (stuck open). Replacement is straightforward on most engines.
How the thermostat works
The thermostat contains a wax element that expands when heated. When the engine reaches operating temperature, the wax pushes the valve open, allowing coolant to flow through the radiator. When the engine cools, the valve closes to speed warm-up.
- Closed at cold start — allows engine to reach operating temperature quickly
- Opens progressively between 82 °C and 95 °C (varies by engine)
- Full flow at operating temperature — coolant circulates through the radiator
- Closing again during motorway driving in cold weather maintains stable temperature
- Some modern engines use electrically heated thermostats for precise control
Signs of thermostat failure
Thermostat failure is usually binary — stuck open or stuck closed. Both conditions are easy to diagnose from the temperature gauge behaviour.
- Stuck closed: rapid overheating, steam from bonnet, temperature gauge in the red
- Stuck open: engine never reaches operating temperature, heater blows lukewarm air
- Erratic temperature gauge readings — cycling between hot and cold
- Increased fuel consumption (engine management enriches mixture when cold)
- Check engine light or cooling system fault code
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Stuck open or stuck closed? Learn how thermostats work and when to replace yours.
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Frequently Asked Questions
What does the engine thermostat do?
It regulates coolant flow to maintain optimal engine temperature. It stays closed when the engine is cold (for fast warm-up) and opens at operating temperature to allow coolant circulation.
How do I know if my thermostat is stuck?
Stuck open: engine takes too long to warm up, heater blows cool air. Stuck closed: engine overheats quickly, temperature gauge spikes.
Should I replace the thermostat gasket?
Yes — always fit a new gasket or O-ring when replacing the thermostat to prevent coolant leaks.
How much does thermostat replacement cost?
The price of a thermostat depends on your exact vehicle and the brand you choose. Enter your registration above to see parts confirmed to fit, then compare prices from the brands we stock on this page. Free UK delivery on orders over £99.
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Blower Motor Buying GuideNo air from the vents? Blower motor failure diagnosis, resistor module faults, and replacement guide.
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