Your boiler's main internal computer has developed a serious communication error and cannot operate, often because it has been exposed to water or has suffered an electrical failure.

Gas valve mode switch not set to 'NORM' or water temperature low/gas pressure issues.

What causes this fault?

This fault occurs when the boiler's main control board suffers a critical internal failure and can no longer process data to operate the system. The most common cause is water ingress from a leaking internal component dripping onto the electronics, or a power surge that has damaged the sensitive electrical tracks on the board.