BoilerFaultCodesGlow-worm d.91 Fault Code
Also written as D91 — same fault.
The Glow-worm d.91 fault code means: your boiler is currently trying to synchronize its internal clock with a radio signal from an external transmitter, which is a standard background process and not a fault.. It appears on 9 Glow-worm models in our database.
Technical description: Status DCF77
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Glow-worm models that show the d.91 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm d.91 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 7 July 2026