BoilerFaultCodes D.4 Low
Boiler Fault Code D.4
Also written as D4 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code D.4 appears across 1 boiler brand and 4 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 showing the live temperature reading of the sensor responsible for keeping your hot water pre-heated. (As reported on Vaillant ecoTEC)
Pick your boiler brand for the D.4 fix
Source
Information for the D.4 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