d3 Medium

Alpha d3 Fault Code

Also written as D3 — same fault.

The Alpha d3 fault code means: your boiler is struggling to accurately detect or manage the temperature of the hot water it is producing, which may result in your taps running cold or the water being hotter than intended.. It appears on 8 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: Domestic hot water temperature leaving the boiler
Typical repair cost £120-220 (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 d3 fault code

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

Source

Alpha d3 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