BoilerFaultCodesGlow-worm F4 Fault Code
The Glow-worm F4 fault code means: your boiler has detected an issue with the sensor that monitors hot water temperature, meaning it cannot safely heat water for your taps.. It appears on 7 Glow-worm models in our database.
Technical description: Domestic hot water thermistor fault
What causes the Glow-worm F4 fault?
This fault occurs when the internal sensor responsible for measuring the temperature of your tap water fails or sends an incorrect signal to the main control board. The most common reason is that the sensor has physically degraded over time or developed a short circuit, preventing the boiler from knowing how hot the water is. In some cases, the issue may simply be loose or damaged wiring between the sensor and the boiler's computer.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Glow-worm models that show the F4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F4 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 27 April 2026