Boiler Fault Code D.078

Also written as D078 — same fault.

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

Fault code D.078 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 has detected that the water being used for your hot taps is reaching a higher temperature than it should be, so it is temporarily limiting itself to prevent overheating. (As reported on Vaillant ecoTEC Pro)