Your boiler's main control panel has lost connection with the internal computer that runs it, meaning the boiler cannot receive commands.

Flue sensor open circuit - check connections to flue sensor are in place

What causes this fault?

This fault occurs when the data link between the boiler's display screen and the main internal computer is broken or interrupted. The most common reason is a loose or damaged wiring connection inside the casing, though it can also be caused by moisture affecting the delicate electronic circuit boards.