Your boiler has paused its heating process because an external control or smart accessory is preventing it from firing up.

Accesory Module is blocking burner operation

What causes this fault?

This code occurs when an external control device, such as a smart thermostat or a zone valve, is sending a signal to the boiler that prevents it from firing. The boiler is technically ready to work, but it is being 'blocked' by a command from an accessory that may be malfunctioning or incorrectly wired. In many cases, this is caused by a communication failure between the wireless receiver and the boiler itself.