4-b5 Low

Worcester Bosch 4-b5 Fault Code

Also written as 4B5 — same fault.

The Worcester Bosch 4-b5 fault code means: your boiler is notifying you that its internal protection against freezing has been activated or deactivated, which is a normal safety monitoring function.. 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: Device frost protection: OFF: switched off, ON: switched on.

DIY fix possible

Check your home temperature is above 5°CEnsure the boiler power supply remains switched onMonitor the display to see if the code clears once the unit reaches a safe temperature

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Worcester Bosch models that show the 4-b5 fault code

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

Source

Worcester Bosch 4-b5 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