d12 High

Boiler Fault Code d12

Also written as D12 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code d12 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is reporting an issue with the water circulation pump speed, which is preventing it from effectively moving heat around your home. (As reported on Alpha Evoke 10 / 18)

Pick your boiler brand for the d12 fix

Alpha d12 5 models

Source

Information for the d12 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 7 July 2026