BoilerFaultCodesGlow-worm -9OC Fault Code
Also written as 9OC — same fault.
The Glow-worm -9OC fault code means: your boiler has detected a problem with the sensor that monitors hot water temperature, likely due to a loose connection or a fault in the internal wiring.. It appears on 6 Glow-worm models in our database.
Technical description: Check that the DHW thermistor wires are unconnected and are not faulty or trapped.
What causes the Glow-worm -9OC fault?
This fault occurs when the boiler's control board loses communication with the sensor responsible for monitoring your hot water temperature. The most common reason is a loose electrical connection or a break in the internal wiring, preventing the system from knowing if the water is at a safe temperature. Because the boiler cannot verify the heat level, it stops providing hot water as a safety precaution.
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 -9OC fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm -9OC 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