Boiler Fault Code D.025

Also written as D025 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026

Fault code D.025 appears across 1 boiler brand and 7 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 receiving a signal from your room thermostat or controller to heat your water, and it is simply showing you that this function is active. (As reported on Vaillant ecoTEC Pro 30)