BoilerFaultCodesGlow-worm F09 Fault Code
The Glow-worm F09 fault code means: your boiler has lost track of the system's water pressure because the internal sensor is either faulty or disconnected, causing the heating to stop working for safety.. It appears on 1 Glow-worm model in our database.
Technical description: Water pressure sensor fault
What causes the Glow-worm F09 fault?
This fault occurs when the boiler's internal computer can no longer detect the water pressure inside the system. The most common reason is that the pressure sensor has developed an internal electrical failure or has become blocked with debris, preventing it from sending a signal. In some cases, the thin wires connecting the sensor to the main control board may have come loose or suffered damage.
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 F09 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F09 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