BoilerFaultCodesGlow-worm F.12 Fault Code
Also written as F12 — same fault.
The Glow-worm F.12 fault code means: your boiler has lost its ability to monitor the temperature of your hot water cylinder because a sensor has developed an electrical fault.. It appears on 3 Glow-worm models in our database.
Technical description: Cylinder temperature sensor short circuit
What causes the Glow-worm F.12 fault?
This fault occurs when the electrical monitoring system detects a short circuit in the sensor that measures the heat of your hot water tank. The most common reason is that the internal components of the sensor have failed, or the wiring connecting it to the boiler has become damaged or pinched. As a result, the boiler can no longer safely determine if the water is at the correct temperature and will stop heating the cylinder as a safety precaution.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the F.12 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F.12 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