Your boiler's main control board is receiving an incorrect electrical signal from the flame sensor, meaning the system cannot safely confirm if the burner is lit.

Flame monitor input signal is outside the limits (0 or 5 V): electronics defective

What causes this fault?

This fault occurs when the boiler's central brain receives a conflicting electrical signal regarding the presence of a flame, causing it to shut down for safety. The most common reason is an internal electronic failure within the main control board or a short circuit in the wiring connecting the sensors. Essentially, the boiler can no longer trust its own sensors to tell if it is burning gas safely.