Your boiler has detected an electrical safety issue where the power supply is not properly grounded or the internal control board has failed.

Mains supply not correctly earthed. Control unit or earth fault

What causes this fault?

This fault occurs when the boiler's internal computer detects that the electrical supply is missing a secure connection to the ground. The most common reason is a loose or corroded wire within the boiler itself, or a critical failure of the main control board which prevents it from sensing the electricity correctly.