Your boiler has lost its connection to an internal sensor or controller, meaning it cannot communicate properly to start the heating process.

Defective cable connection.

What causes this fault?

This fault occurs when the boiler's main control board loses the electrical signal from one of its internal components. The most common reason is a loose or damaged wiring connection that prevents the system from confirming it is safe to fire up. It can also be triggered by moisture affecting the plugs or a hardware failure within the printed circuit board itself.