d2 Medium

Alpha d2 Fault Code

Also written as D2 — same fault.

The Alpha d2 fault code means: your boiler is currently monitoring the temperature of the water as it leaves the unit to heat your home, but it is detecting an abnormal reading that is preventing it from firing up correctly.. 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: Central heating 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 d2 fault code

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

Source

Alpha d2 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