BoilerFaultCodesGlow-worm F8 Fault Code
The Glow-worm F8 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing the system from heating correctly.. It appears on 1 Glow-worm model in our database.
Technical description: Central Heating return thermistor fault.
What causes the Glow-worm F8 fault?
This fault occurs when the sensor responsible for monitoring the temperature of the water returning to the boiler sends an incorrect signal or fails entirely. It is usually caused by the internal components of the sensor wearing out over time, or by damaged wiring that prevents the boiler's computer from receiving accurate data.
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 F8 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F8 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