Your boiler's electronic control system is failing to correctly detect whether the burner is lit, so it has safely shut down to prevent gas from building up.

Flame detector input signal is outside the limits (0 or 5 V). Electronics defective

What causes this fault?

This fault occurs when the boiler's main control board receives an impossible or conflicting electrical signal from the flame sensing system. The most common reason is an internal electronic failure within the printed circuit board (PCB) itself or a total malfunction of the sensing electrode that monitors the burner.