Your boiler has detected a problem with the sensor that monitors the temperature of the exhaust gases, and it has likely shut down as a safety precaution.

Temperature sensors faulty (DHW and CH sensors)

What causes this fault?

This fault occurs when the sensor inside the flue detects that the exhaust gases are getting too hot, or if the sensor itself has failed and is sending incorrect data to the control board. It acts as a vital safety trip to prevent the boiler from overheating and damaging internal components. Common causes include a buildup of debris in the heat exchanger or a simple electrical failure within the sensor's internal wiring.