d1 Medium

Alpha d1 Fault Code

Also written as D1 — same fault.

The Alpha d1 fault code means: your boiler is displaying its current flame strength reading, which is usually a status indicator rather than a fault, but it may suggest the system is struggling to establish or maintain a stable flame.. It appears on 9 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: Flame signal mA x 10 (approx)
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 d1 fault code

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

Source

Alpha d1 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