Your boiler has stopped working because it has detected a technical fault with the sensor that monitors the heat of the exhaust gases.

Burner blocked - Flue gas temperature sensor short circuit

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect electrical signal from the sensor that monitors the temperature of the exhaust fumes. The most common reason is that the sensor itself has suffered an internal failure or the electrical wiring connecting it to the main control board has become damaged or loose. Because the boiler can no longer safely monitor how hot the waste gases are, it shuts down immediately as a safety precaution.