F.91 Medium

Glow-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 7 Glow-worm models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Short circuit: Cylinder temperature sensor
Typical repair cost £120-200 (including labour)

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.

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 7 July 2026