4-b6 Low

Worcester Bosch 4-b6 Fault Code

Also written as 4B6 — same fault.

The Worcester Bosch 4-b6 fault code means: your boiler has detected that the internal water temperature is very low and has activated its built-in frost protection mode to prevent freezing.. 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: Temperature value for the system frost protection: 0...5...10 °C.
Typical repair cost £0-80 (including labour)

DIY fix possible

Ensure the boiler's electrical power remains switched onCheck that the room thermostat or programmer is calling for heatAllow the boiler to run its automatic anti-freeze cycle until the temperature risesEnsure any external pipework is properly insulated with lagging

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Worcester Bosch models that show the 4-b6 fault code

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

Source

Worcester Bosch 4-b6 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