Your boiler has experienced a power disruption while it was already trying to report a serious internal error, causing it to lock itself for safety.

Control fault: Voltage failure when malfunction causes interlock.

What causes this fault?

This fault occurs when the boiler experiences a sudden power cut or electrical surge at the exact moment it was trying to register a different internal error. This creates a conflict in the boiler's memory, as the control board cannot safely confirm why the system shut down. It is essentially a safety 'double-lock' that prevents the boiler from firing up until the electrical integrity of the control unit is verified.