Your boiler's internal computer has detected an electrical fault with the flame sensor and has shut down for safety.

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 a corrupt electrical signal from the flame sensing system, making it unable to verify if the burner is safely lit. The most common reason is a communication failure within the internal circuitry, often caused by a short circuit or a component failure on the printed circuit board itself.