BoilerFaultCodesGlow-worm S.91 Fault Code
Also written as S91 — same fault.
The Glow-worm S.91 fault code means: your boiler has been accidentally set to a demonstration mode used in showrooms, meaning it is not actually firing up to heat your home or water.. It appears on 16 Glow-worm models in our database.
Technical description: Maintenance: Demo mode
DIY fix possible
Locate the boiler's installation manualNavigate to the diagnostic menu on the display screenFind the 'Demo Mode' setting and switch it to 'Off' or '0'
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like S.91: no call-out fees, no surprise bills.
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Glow-worm models that show the S.91 fault code
Click a model for the full fault code list and model-specific repair notes.
- Betacom4
- Easicom3 Combi Repair: £80-120
- Easicom3 System Repair: £80-150
- Energy Repair: £0-£100
- Energy Regular Repair: £80-120
- Energy System Repair: £80-150
- Energy2 30c
- Energy7 Combi Repair: £80-120
- Energy7 Regular Repair: £80-150
- Energy7 System
- Essential Repair: £80-150
- Flexicom 24c / 28c Repair: £80-120
- Sustain 12s Repair: £80-150
- Ultimate 3 25R Repair: £80-150
- Ultimate3 Combi Repair: £80-150
- Ultimate3 System Repair: £80-150
Source
Glow-worm S.91 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
- Glow-worm Betacom4 manual (PDF)
- Glow-worm Easicom3 Combi manual (PDF)
- Glow-worm Easicom3 System manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026