High Lockout
Boiler overheat thermostat on Worcester Bosch Greenstar Utility 18/25

Your boiler has become too hot and has automatically shut itself down to prevent damage or safety issues.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Reset Button - Boiler overheat thermostat

What causes this fault?

This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety switch to cut the power. The most common reasons include a failing pump that isn't moving water away quickly enough or a buildup of limescale and sludge that restricts flow through the heat exchanger.

This issue is more frequent in autumn when heating systems are first turned on after a long summer break, as stagnant debris can settle and block the pump.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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