Boiler Fault Code d.21

Also written as D21 — same fault.

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

Fault code d.21 appears across 1 boiler brand and 16 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning This code is simply an information message telling you whether your boiler's 'fast hot water' feature is currently turned on or off, rather than signaling a fault. (As reported on Vaillant ecoTEC plus System)