Your boiler is showing this code simply because the reset button was held down for too long or pressed by mistake, which has confused the internal software.

Mains power error - electrical power interruption

What causes this fault?

This fault occurs when the boiler's control board detects that the reset button has been compressed for a prolonged period, typically more than 15 seconds. It usually happens if the button is pressed too hard, held down in an attempt to clear another error, or if the button has physically stuck inside the control panel. The boiler enters this state as a safety precaution because it cannot distinguish between a genuine reset attempt and a hardware malfunction.