Your boiler is having trouble communicating with the pump that heats your hot water cylinder, meaning you may experience a lack of hot water.

Cylinder charging pump via accessory module

What causes this fault?

This fault occurs when the boiler's main control board loses its connection with the external pump responsible for heating your hot water tank. It is usually caused by a loose electrical connection, a failing communication module, or the pump itself developing an internal electronic fault that prevents it from responding to the boiler.