Your boiler's electronic control board is having trouble communicating with the flame sensor, meaning the system cannot safely confirm if the burner is lit.

Electronic fault in flame system (non–plausible flame signal)

What causes this fault?

This fault occurs when the boiler's main control board loses the ability to correctly process the signal from the flame sensor. The most common reason is an internal electrical failure on the printed circuit board, though it can also be triggered by damaged wiring or worn ignition electrodes that are no longer detecting the flame properly.