Your boiler has detected that the electrical wiring coming from your home’s power supply matches the wrong terminals, meaning the power flow is backwards.

Live and Neutral reversed

What causes this fault?

This fault occurs when the boiler detects that the live and neutral electrical connections have been swapped at the power source or within the internal wiring. It is most commonly caused by recent electrical work in the home or an incorrectly wired plug or fused spur, which poses a safety risk to the internal electronics. Because British boilers are polarity-sensitive, the system will prevent operation until the wiring is physically corrected.