BoilerFaultCodesGlow-worm F9 Fault Code
The Glow-worm F9 fault code means: your boiler has detected an issue with its internal sensor that monitors water pressure, preventing it from heating up.. It appears on 5 Glow-worm models in our database.
Technical description: Water pressure sensor fault.
What causes the Glow-worm F9 fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors internal water pressure. The most common reason is that the sensor has developed an electrical failure or has become blocked with system debris, preventing it from sending a steady signal. In some cases, the issue is caused by loose or damaged wiring connecting the sensor to the main computer.
DIY fix possible
Check the pressure gauge on the front of the boiler is between 1.0 and 1.5 barIf the pressure is low, use the filling loop underneath to top the system upReset the boiler using the power or reset button to clear the fault
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Gas Safe Register →Glow-worm models that show the F9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F9 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