Boiler Fault Code d.91

Also written as D91 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026

Fault code d.91 appears across 2 boiler brands and 17 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is currently trying to synchronize its internal clock with a radio signal from a time transmitter, which is a standard background process and does not affect your heating or hot water. (As reported on Vaillant ecoFIT Pure 835)