b4 Medium

Boiler Fault Code b4

Also written as B4 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code b4 appears across 3 boiler brands and 6 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has lost the connection to its outdoor temperature sensor and is assuming a fixed temperature of 0°C to keep your heating running safely. (As reported on Viessmann Vitodens 222-F)

Pick your boiler brand for the b4 fix

Ideal b4 1 model Viessmann b4 4 models Worcester Bosch b4 1 model

Source

Information for the b4 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026