Your boiler has stopped working because it cannot accurately measure its own internal temperature due to a broken or disconnected sensor wire.

Burner blocked - Boiler temperature sensor lead break

What causes this fault?

This fault occurs when the boiler's main internal thermometer stops sending a steady electrical signal to the control board. The most common reason is a physical break in the thin copper wiring or a connector that has vibrated loose, preventing the boiler from knowing its own temperature. Because the boiler cannot safely monitor if it is overheating, it shuts down the burner as a safety precaution.