Your boiler has detected that the internal temperature sensors have been wired or installed in the wrong positions, preventing it from heating safely.

Flow and return sensors NTC1 and NTC2 reversed

What causes this fault?

This fault occurs when the boiler's control board detects that the water temperature readings are backwards, usually following a recent repair or component replacement. It is physically caused by the flow and return temperature sensors being plugged into the wrong ports on the main circuit board or clipped onto the incorrect internal pipes. Because the boiler cannot accurately track how heat is moving through the system, it shuts down to prevent overheating.