Boiler Fault Code 5b:1

Also written as 5B1 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026

Fault code 5b:1 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has detected that the hot water cylinder is incorrectly positioned in the system's plumbing layout, which may cause inefficient heating of your water. (As reported on Viessmann Vitodens 300-W)