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

Accessory Module is blocking burner operation

What causes this fault?

This fault occurs when an external control device or a specialized plug-in module sends a signal that overrides the boiler's ability to start. It usually happens because a component like a programmer or zone control unit has developed a communication error or has detected a safety issue that requires the burner to stay off. In some cases, a loose connection or moisture in the wiring between these external parts and the boiler is the primary trigger.