Your boiler has shut down because it cannot accurately measure the temperature of the exhaust gases, causing the burner to stop 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 a physical break in the sensor's internal circuitry or a loose connection in the wiring harness. For safety, the boiler shuts down because it can no longer verify that the exhaust temperatures are within safe limits.