Your boiler is having trouble communicating with an add-on control or an external smart home accessory, which may affect its ability to run efficiently.

Control mode - Communication fault-ext. extension

What causes this fault?

This fault occurs when the boiler's main control board loses the digital handshake with an external accessory, such as a smart home interface or an expansion module. The most common reason is a loose connection in the low-voltage wiring or a hardware failure within the add-on component itself. Without this communication link, the boiler can no longer receive instructions on how hard it needs to work to heat your home.