4-b3 Low

Worcester Bosch 4-b3 Fault Code

Also written as 4B3 — same fault.

The Worcester Bosch 4-b3 fault code means: your boiler is notifying you that the settings for its automatic outdoor temperature sensors are incorrectly configured or out of their allowed range.. 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: End point of the heating curve for weather-compensated control: 20... 90 °C.
Typical repair cost £0-80 (including labour)

DIY fix possible

Navigate to the service menu on the control panelLocate the weather compensation settingsAdjust the heating curve end point to a value between 20°C and 90°CPress the reset button to clear the notification

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Worcester Bosch models that show the 4-b3 fault code

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

Source

Worcester Bosch 4-b3 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