BoilerFaultCodesWorcester Bosch 9.b Fault Code
Also written as 9B — same fault.
The Worcester Bosch 9.b fault code means: your boiler's fan is not spinning at the correct speed, which prevents the system from safely starting the combustion process.. It appears on 1 Worcester Bosch model in our database.
Technical description: Actual fan speed (Hz)
What causes the Worcester Bosch 9.b fault?
This fault occurs when the boiler's internal computer is unable to confirm that the fan is spinning at the required speed to safely clear waste gases. It is usually caused by a physical failure of the fan motor, a loose connection in the wiring, or a fault on the main control board that prevents it from reading the fan's speed correctly.
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 9.b fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 9.b 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