BoilerFaultCodesWorcester Bosch 9F855 Fault Code
The Worcester Bosch 9F855 fault code means: your boiler has stopped working because it cannot light the burner, which most often happens when your oil tank is empty or there is an issue with the oil supply.. It appears on 1 Worcester Bosch model in our database.
Technical description: Burner lockout indicated by the control panel LCD display; burner fails to establish a normal firing pattern or flame failure occurs.
What causes the Worcester Bosch 9F855 fault?
This fault happens because the burner has failed to establish a flame after several attempts, causing the boiler to shut down for safety. The most common cause is simply a lack of fuel reaching the burner, either because the oil tank is empty or there is a blockage in the supply line. It can also be caused by a dirty sensor that can no longer 'see' the flame or a worn-out nozzle that isn't spraying fuel correctly.
This fault is particularly common during the first cold snap of autumn when oil levels may be low following the summer or when thickened 'waxing' occurs in the oil during extremely cold winter spells.
DIY fix possible
Check the level of oil in your fuel tank to ensure it hasn't run dry.Verify that the oil supply valves to the boiler are fully open.Press the reset button on the control panel once to see if the burner restarts.
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Gas Safe Register →Worcester Bosch models that show the 9F855 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 9F855 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