Low
4-b4 on Worcester Bosch Greenstar 2000

Your boiler is notifying you that there is a settings error or a communication issue with your outdoor temperature sensor or heating curve configuration.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Base point of the heating curve for weather-compensated control: 20... 90 °C.

From this model's manual
Verified against the Worcester Bosch Greenstar 27cdi installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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