Your boiler is confused because it doesn't know which type of heating system it is supposed to be controlling.

What causes this fault?

This fault occurs when the boiler's internal memory loses track of the specific heating layout it is supposed to manage, such as whether it is heating just radiators or a hot water cylinder too. The most common reason is a communication error between the main control board and the coding plug, which acts like a small identity chip for the appliance. Without this clear 'identity,' the boiler stops operating because it no longer knows which internal components to activate.