Boiler Fault Code d.09

Also written as D09 — same fault.

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

Fault code d.09 appears across 1 boiler brand and 20 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 displaying the target temperature being requested by your external room thermostat or smart control system; this is a status message rather than a breakdown. (As reported on Vaillant ecoFIT Pure 425)