BoilerFaultCodesGlow-worm F.91 Fault Code
Also written as F91 — same fault.
The Glow-worm F.91 fault code means: your boiler is having trouble communicating with the temperature sensor in your hot water cylinder, which means it cannot properly monitor or heat your stored water.. It appears on 14 Glow-worm models in our database.
Technical description: Short circuit: Cylinder temperature sensor
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 F.91 fault code
Click a model for the full fault code list and model-specific repair notes.
- EASICOM 3 25r Repair: £120-220
- Energy Repair: £120-220
- Energy Regular Repair: £120-200
- Energy System Repair: £120-200
- Energy2 30c Repair: £120-220
- Energy7 Combi Repair: £120-220
- Energy7 Regular Repair: £120-200
- Energy7 System Repair: £120-220
- Sustain 12s Repair: £120-200
- Sustain 15s Repair: £120-200
- Sustain 18s Repair: £120-200
- SUSTAIN 30c Repair: £120-220
- SUSTAIN 35c Repair: £120-220
- Ultimate 3 25R Repair: £120-220
Source
Glow-worm F.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 31 August 2026