There is a communication error between your boiler and its controls, often caused by an electrical wiring fault or a heavy load on the internal data connection.

eBUS undervoltage: Short-circuit on eBUS input, eBUS overload or two power supplies with different polarities on the eBUS

What causes this fault?

This fault occurs when the electrical communication line between your boiler and its thermostat becomes overloaded or suffers a short circuit. It is often caused by damaged wiring, an incompatible external controller, or a component failure on the main circuit board that prevents the system from sending clear signals. Essentially, the boiler can no longer 'talk' to its controls, so it shuts down as a safety precaution.