BoilerFaultCodesGlow-worm FA9 Fault Code
The Glow-worm FA9 fault code means: your boiler's internal communication system has lost power, meaning the different components cannot talk to each other to start the heating process.. It appears on 6 Glow-worm models in our database.
Technical description: eBUS fault – voltage too low
What causes the Glow-worm FA9 fault?
This fault occurs when the internal communication line, which allows the display and controls to talk to the main brain of the boiler, loses electrical power. The most common reason is a short circuit in the wiring or a failing printed circuit board that is no longer able to distribute the correct voltage. It can also be triggered if an external smart thermostat or controller draws too much power from the system.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the FA9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm FA9 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026