Boiler Fault Code D.4

Also written as D4 — same fault.

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

Fault code D.4 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is displaying a status reading for the warm start sensor temperature rather than an error, which is part of the system's normal operational monitoring. (As reported on Vaillant ecoTEC plus Regular)