Your boiler has shut down because it has detected an unsafe increase in temperature and has triggered a safety cut-out to prevent damage.

Safety Thermostat Operated (Overheat of the primary water)

What causes this fault?

This fault occurs when the water inside the boiler reaches an unsafe temperature, forcing a safety switch to trip and shut the system down. The most common reasons are a lack of water flow caused by a failing pump, a blockage in the pipework, or a buildup of air within the heat exchanger.