Your boiler has temporarily stopped heating because an external add-on or control accessory is preventing it from firing up.

Accesory Module is blocking burner operation

What causes this fault?

This fault occurs when an external control device, such as an additional wiring center or a specific zone controller, tells the boiler to stop running for safety or logic reasons. The most common reason is a communication error between the main boiler brain and an add-on expansion module that manages extra heating zones. It suggests that while the boiler itself may be healthy, the external parts controlling it have detected a conflict or a hardware failure.