BoilerFaultCodesWorcester Bosch G9 Fault Code
The Worcester Bosch G9 fault code means: your boiler has detected that hot water is escaping through the safety overflow pipe, usually because the system is overheating or a valve has failed.. It appears on 1 Worcester Bosch model in our database.
Technical description: Hot water fault over-flow pipe
What causes the Worcester Bosch G9 fault?
This fault occurs when the internal pressure within your boiler gets too high, forcing the safety valve to open and release water to prevent damage. The most common reasons are a loss of air in the expansion vessel or a buildup of limescale preventing the safety valve from closing properly. Since the system has lost water and pressure, the boiler shuts down to protect itself from overheating.
This issue often surfaces in autumn when the heating is turned on for the first time after summer, as the expansion vessel may have lost its charge during the months of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Worcester Bosch models that show the G9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch G9 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026