How much does it cost to fix boiler error code d21?

Typical repair cost for the boiler d21 fault is £120-250 including labour.

d21 Medium

Boiler Fault Code d21

Also written as D21 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 7 July 2026

Fault code d21 appears across 1 boiler brand and 2 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is displaying a code that is not currently assigned to a specific fault, which usually points to a system communication glitch or a software error. (As reported on Alpha E-Tec Plus 10 / 18)

Pick your boiler brand for the d21 fix

Alpha d21 2 models

Source

Information for the d21 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