BoilerFaultCodes D.12 High
Boiler Fault Code D.12
Also written as D12 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code D.12 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 having trouble communicating with the pump that heats your hot water cylinder, meaning you may experience a lack of hot water. (As reported on Vaillant ecoTEC)
Pick your boiler brand for the D.12 fix
Source
Information for the D.12 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026