BoilerFaultCodesAlpha 0A21 Fault Code
The Alpha 0A21 fault code means: your boiler's central control computer has encountered an internal error and cannot communicate properly, meaning the system has shut down for safety.. It appears on 3 Alpha models in our database.
Technical description: Main PCB fault.
What causes the Alpha 0A21 fault?
This fault occurs when the boiler's internal control board suffers a hardware failure and can no longer process data correctly. The most common reason is damage to the electrical components on the board, often caused by natural wear and tear over time or an accidental electrical surge.
This fault is slightly more common during the winter months when the boiler is under constant heavy use, which can lead to overheating or failure of older electrical components.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Alpha models that show the 0A21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Alpha 0A21 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 27 April 2026