Your boiler is notifying you that it has been configured to be controlled by a single external timer or thermostat wire rather than a smart system.

What causes this fault?

This code appears because the boiler has been electronically told to look for a single basic control wire to turn the heating on and off. It often happens after a power cut, a system reset, or if a new smart thermostat has been installed incorrectly, causing the boiler's internal settings to mismatch the physical wiring. Rather than a broken part, it is simply the boiler asking for clarification on how it should receive its instructions.