b4 High

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 10 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler's internal computer is having trouble processing electrical signals correctly, which is preventing it from running its safety and efficiency tests. (As reported on Viessmann Vitodens 333-F)

Pick your boiler brand for the b4 fix

Ideal b4 1 model Viessmann b4 6 models Worcester Bosch b4 3 models

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 31 August 2026