Your boiler has detected an internal communication error or a disconnected jumper wire, which usually prevents the preheat function or the boiler from operating correctly.

Go to section ‘L’: Check continuity of loop on A10 connector.

What causes this fault?

This fault occurs when the boiler's main control board loses its connection to a specific internal bypass wire that tells it how to manage the preheat function. It is usually caused by a loose connection at the 'A10' plug or a physical break in the wiring harness, which prevents the boiler from completing its safety checks. In some cases, the internal electronics on the printed circuit board may have failed, meaning it can no longer 'see' the loop even if the wire is intact.