Boiler Fault Code d.66

Also written as D66 — same fault.

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

Fault code d.66 appears across 1 boiler brand and 20 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is simply indicating that the 'warm start' feature is currently active, which keeps a small amount of water pre-heated so your hot water arrives at the tap more quickly. (As reported on Vaillant ecoFIT Pure 412)