Your boiler has detected a broken connection or a fault with the sensor that monitors the temperature of the exhaust gases, causing the system to stop working for safety.

Burner blocked: Flue gas temperature sensor break

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors the heat of the exhaust gases. The most common reason is that the sensor has physically failed or developed a break in the internal wiring, though it can also be triggered if the sensor has become dislodged from its mounting clip. For safety, the boiler shuts down immediately to ensure the exhaust system does not overheat.