BoilerFaultCodes F.91 Medium
Boiler Fault Code F.91
Also written as F91 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code F.91 appears across 3 boiler brands and 28 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 trouble communicating with the temperature sensor in your hot water cylinder, which means it may not be able to heat your taps or shower correctly. (As reported on Vaillant ecoFIT Pure 825)
Pick your boiler brand for the F.91 fix
Source
Information for the F.91 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