BoilerFaultCodesVokera AL21 Fault Code
The Vokera AL21 fault code means: your boiler has shut down because a safety device or an external controller has detected an issue and stopped the system from running.. It appears on 8 Vokera models in our database.
Technical description: External device fault (UHT/CPA)
What causes the Vokera AL21 fault?
This fault occurs when an external safety device or a limit thermostat detects that the boiler is getting too hot, triggering an emergency shutdown to prevent damage. It is often caused by a lack of water flow through the heat exchanger or a loose connection in the wiring that links the boiler to its external controls. In some cases, the sensor itself may have failed and is sending an incorrect signal to the main control board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Vokera models that show the AL21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vokera AL21 fault code information on this page is sourced and cross-checked against the official Vokera service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026