BoilerFaultCodesWorcester Bosch 9F856 Fault Code
The Worcester Bosch 9F856 fault code means: your boiler has stopped working because it has been reset too many times while trying to clear a burner fault, and it has now locked itself for safety.. It appears on 1 Worcester Bosch model in our database.
Technical description: If burner lockout is reset 5 times in one demand period using the fascia reset button the controls will lockout with a new code: 9F 856. This is to protect the burner components against over use of the reset and requires a power down of the appliance to reset.
What causes the Worcester Bosch 9F856 fault?
This fault occurs when the boiler's safety system permanently locks itself to prevent damage after the manual reset button has been pressed too many times. It is triggered by an underlying failure in the burner system, such as a blocked oil nozzle or a faulty ignition component, which prevents the flame from lighting correctly. Once this specific code appears, the boiler will no longer respond to simple resets and requires a professional intervention to clear the memory.
This fault is frequently seen in autumn and winter when the oil in the tank or supply lines becomes more viscous due to the cold, potentially leading to blockages that trigger the initial burner failure.
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 9F856 fault code
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Source
Worcester Bosch 9F856 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