BoilerFaultCodes D.021 Low
Boiler Fault Code D.021
Also written as D021 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code D.021 appears across 1 boiler brand and 3 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, which keeps a small amount of water pre-heated for faster hot water taps, is currently active. (As reported on Vaillant ecoTEC plus)
Pick your boiler brand for the D.021 fix
Source
Information for the D.021 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