BoilerFaultCodesWorcester Bosch A9 Fault Code
The Worcester Bosch A9 fault code means: your boiler is struggling to correctly flow water through the system, which is preventing it from heating up properly.. It appears on 1 Worcester Bosch model in our database.
Technical description: Diaphragm
What causes the Worcester Bosch A9 fault?
This fault occurs when the rubber diaphragm inside the diverter valve perishes or tears, preventing the boiler from detecting when you turn on a hot tap. Because the internal mechanism cannot move correctly to divert heat, the boiler fails to fire up even though the water is running. Over time, the rubber naturally loses its elasticity or develops pinholes due to constant stretching and contact with system debris.
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 A9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch A9 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