Your boiler has lost communication with its external controls, such as your thermostat, often due to an electrical wiring error or interference.

eBUS undervoltage | Short circuit on eBUS, overload on eBUS or two power sources on eBUS with different polarity

What causes this fault?

This fault occurs when there is an electrical conflict or a lack of power on the digital communication line that connects your boiler to your thermostat. It is usually caused by wires being accidentally crossed during installation, a physical short circuit in the cabling, or an internal electronic failure in the control module. Effectively, the boiler can no longer 'hear' what the thermostat is telling it to do, so it shuts down for safety.