Your boiler has likely overheated, causing a safety switch to trip and shut the system down to protect itself.

Safety Thermostat Operated

What causes this fault?

This fault occurs when the water temperature inside your boiler gets too high, triggering a safety switch to prevent damage. The most common reason is a lack of water flow, often caused by a failing pump or a blockage within the main heat exchanger. It can also happen if there is trapped air in the system or if the internal safety sensor itself has developed a fault.