Your boiler has temporarily stopped providing heating or hot water because an external control or smart accessory is preventing it from firing up.

Accesory Module is blocking burner operation

What causes this fault?

This fault occurs when an external device connected to the boiler, such as a smart thermostat or a wireless receiver, sends a signal to stop the burner from firing. The most common reason is a loss of communication between your thermostat and the boiler, often caused by dead batteries or a signal interference. Essentially, the boiler is waiting for a 'go' signal from your controls that never arrives or is being actively blocked.