BoilerFaultCodesGlow-worm 12 Fault Code
The Glow-worm 12 fault code means: your boiler is having trouble communicating with its control screen or your thermostat, which is preventing it from receiving your heating instructions.. It appears on 8 Glow-worm models in our database.
Technical description: User interface or distant control panel fault
What causes the Glow-worm 12 fault?
This fault occurs when the main computer inside the boiler loses its physical connection to the display panel or the external controller. It is usually caused by a loose wiring connection or a component failure on one of the circuit boards, meaning your instructions can no longer reach the boiler’s ignition system. Think of it like a broken telephone line between your thermostat and the boiler's engine.
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 12 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 12 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