BoilerFaultCodesWorcester Bosch −II− Fault Code
Also written as II — same fault.
The Worcester Bosch −II− fault code means: your boiler is currently running a 15-minute cleaning and safety cycle to fill up its internal drainage system, which is a normal process after a power cut or reset.. It appears on 3 Worcester Bosch models in our database.
Technical description: Trap filling programme is active. The appliance remains at minimum heating output for 15 minutes to ensure the condensation trap is filled.
What causes the Worcester Bosch −II− fault?
This code appears when the boiler needs to refill its internal reservoir that collects waste water, which typically happens after the power has been turned off or the system has been reset. To ensure no exhaust gases escape through the drainage system, the boiler stays at a very low power level for fifteen minutes to safely generate enough condensation to fill the trap. Once this automatic safety cycle finishes, the boiler will return to its normal heating settings without any intervention needed.
DIY fix possible
Wait for 15 minutes for the timer to finishEnsure any thermostat or timer is calling for heatMonitor the display to see if it returns to a normal temperature reading
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Gas Safe Register →Worcester Bosch models that show the −II− fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch −II− 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