d21 Medium

Alpha d21 Fault Code

Also written as D21 — same fault.

The Alpha d21 fault code means: 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.. It appears on 2 Alpha models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Not used
Typical repair cost £120-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Alpha models that show the d21 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Alpha d21 fault code information on this page is sourced and cross-checked against the official Alpha service / installation manuals for each affected model.

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