Your boiler's internal control board is having trouble communicating with an external part or accessory, which may prevent the system from starting up correctly.

What causes this fault?

This fault occurs when the boiler's main control board fails to detect or communicate with an internal switching relay or an external accessory. The most common reason is a loose connection in the wiring or a physical failure of the relay component that manages how the boiler talks to external parts. In some cases, it can indicate that the main circuit board itself can no longer send signals properly.