Your boiler thinks a flame is burning even though it hasn't actually sparked or started, causing it to shut down for safety.

Flame signal without flame present

What causes this fault?

This fault occurs when the boiler's internal sensors mistakenly detect a flame signal while the burner should be off. The most common reason is a faulty or contaminated sensing electrode that is sending a false electrical signal to the control board, preventing the boiler from starting for safety reasons.