Your boiler's main control board has suffered a critical failure, meaning the unit cannot communicate with its internal components to provide heating or hot water.

What causes this fault?

This fault occurs when the internal electrical brain of the boiler suffers a terminal failure, often due to worn-out soldered joints or damaged capacitors on the circuit board. Over time, heat stress and constant electrical cycling cause these components to degrade until the board can no longer process instructions. The most common reason is simply age-related wear, though an external power surge can also occasionally trigger this total shutdown.