Your boiler has detected an internal electronic communication failure or a technical error that requires a deep-level diagnosis by a professional.

ALTERNATING 'H' '2' GO TO FRAME 91

What causes this fault?

This fault occurs when the boiler's internal computer brain loses its ability to send or receive messages across its circuitry. The most common reason is an electronic failure within the main printed circuit board or a loose connection in the internal wiring. This break in communication means the boiler can no longer monitor safety sensors or control the heating process, so it shuts down as a precaution.