Boiler Fault Code D.26

Also written as D26 — same fault.

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

Fault code D.26 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's internal control board is unable to communicate with an optional secondary accessory or pump, causing the system to stop running as a safety precaution. (As reported on Vaillant ecoTEC)