BoilerFaultCodesGlow-worm 37 Fault Code
The Glow-worm 37 fault code means: your boiler has detected that a sensor monitoring the temperature of the water returning to the unit is giving an incorrect reading or has failed, causing the system to stop working as a safety precaution.. It appears on 1 Glow-worm model in our database.
Technical description: The heating return thermistor is too resistant.
What causes the Glow-worm 37 fault?
This fault occurs when the internal control board receives a signal from the return water sensor that is outside of the expected range, suggesting the component is broken or disconnected. The most common reason is that the sensor's internal resistance has degraded over time, though it can also be caused by loose wiring or corrosion on the sensor pins. Because the boiler can no longer accurately monitor the temperature of the water coming back from your radiators, it shuts down to prevent overheating.
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 37 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 37 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