d20 High

Alpha d20 Fault Code

Also written as D20 — same fault.

The Alpha d20 fault code means: your boiler has encountered an undefined system error that is not documented by the manufacturer, which usually suggests a software glitch or a communication failure within the control board.. It appears on 2 Alpha models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Not used
Typical repair cost £120-300 (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 d20 fault code

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

Source

Alpha d20 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