Your boiler has detected that the electrical wiring is connected incorrectly or there is a problem with the mains power supply.

Check the polarity of your system.

What causes this fault?

This fault occurs when the boiler detects that the live and neutral electrical wires have been swapped, or there is an issue with the earth connection. It is most commonly caused by recent electrical work in the home or an incorrectly wired fused spur switch that provides power to the unit. In some cases, a failing internal circuit board can also misread the electrical signals and trigger this safety shutdown.