Your boiler has shut down because a safety sensor has detected that it is getting far too hot, likely due to a blockage or a pump failure.

Maximal flue gas thermostat or heat exchanger maximal thermostat has activated

What causes this fault?

This fault occurs when the water inside the boiler reaches an unsafe temperature or the flue gases become too hot, triggering a safety shut-off to prevent damage. The most common reasons are a failed internal pump that isn't moving water away quickly enough or a blockage in the system from sludge and debris. It can also be caused by an air lock or a closed valve stopping the flow of water through the heat exchanger.