BoilerFaultCodesGlow-worm F12 Fault Code
The Glow-worm F12 fault code means: your boiler has lost communication with the sensor that monitors the temperature of your hot water cylinder, meaning it cannot heat your water correctly.. It appears on 2 Glow-worm models in our database.
Technical description: Cylinder thermistor fault
What causes the Glow-worm F12 fault?
This fault occurs when the boiler can no longer detect a signal from the sensor that monitors the temperature of your hot water tank. The most common reasons are a breakdown of the internal sensor components or a loose connection in the wiring that links the sensor to the boiler's main control board.
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 F12 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F12 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