BoilerFaultCodes F.666 Medium
Boiler Fault Code F.666
Also written as F666 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code F.666 appears across 1 boiler brand and 2 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is having difficulty communicating with a sensor responsible for managing water heating efficiency, which may prevent your hot water from heating up properly. (As reported on Viessmann Vitodens 100-W WB1A)
Pick your boiler brand for the F.666 fix
Source
Information for the F.666 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