4-b4 Low

Worcester Bosch 4-b4 Fault Code

Also written as 4B4 — same fault.

The Worcester Bosch 4-b4 fault code means: your boiler is notifying you that there is a settings error or a communication issue with your outdoor temperature sensor or heating curve configuration.. It appears on 1 Worcester Bosch model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Base point of the heating curve for weather-compensated control: 20... 90 °C.
Typical repair cost £60-120 (including labour)

DIY fix possible

Check that the outdoor sensor is not obstructed or damagedSwitch the boiler off and back on to see if the error clearsEnter the menu to reset the heating curve values if you have recently changed them

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Worcester Bosch models that show the 4-b4 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Worcester Bosch 4-b4 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