BoilerFaultCodesWorcester Bosch P4 Fault Code
The Worcester Bosch P4 fault code means: your boiler is currently running a routine internal cleaning cycle to fill up its condensation trap and is not actually broken.. It appears on 2 Worcester Bosch models in our database.
Technical description: The display shows alternatively the CH flow temperature and P4. The Syphon-fill programme is active. See service function 4.F
What causes the Worcester Bosch P4 fault?
This occurs when the boiler enters an automated maintenance mode to ensure the internal condensate trap is filled with water. It is a protective measure that prevents harmful exhaust gases from leaking into your home through the drainage pipe. The boiler temporarily runs at a lower power setting until the cycle is complete, which usually takes around 15 minutes.
This is more common after a period of inactivity, such as the first time you turn the heating on in autumn or after a summer holiday.
DIY fix possible
Wait approximately 15 to 20 minutes for the 'P4' code to disappear automatically.Do not reset the boiler, as this may restart the timed cycle from the beginning.Once the sequence finishes, the boiler will return to its normal temperature display and resume heating.
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Worcester Bosch models that show the P4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch P4 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 31 August 2026