Your boiler is having trouble 'talking' to an add-on device, such as an extra heating controller or an external sensor module.

Communication error - ext. extension. Control mode

What causes this fault?

This fault occurs when the boiler's main brain loses its connection with an external accessory, such as a specialist wiring centre or an extra heating zone controller. It is usually caused by a loose connection in the communication wiring or a circuit failure within the plug-in module itself, preventing the two devices from sharing data. In some cases, a power surge may have 'scrambled' the communication line, causing the boiler to lose track of the connected equipment.