Your boiler has experienced a power cut or was switched off while it was already showing a different fault, causing it to lock itself out for safety.

Interruption Of Power Supply while in fault mode

What causes this fault?

This fault occurs when the boiler loses electrical power—either due to a local power cut or someone switching off the fused spur—while it was already struggling with a different internal issue. Because the boiler was interrupted while trying to process an existing error, it enters a protective 'lockout' state to prevent you from accidentally running the system in an unsafe condition. It effectively gets stuck in a loop between the old error and the sudden loss of power.