BoilerFaultCodesVokera 21 Fault Code
The Vokera 21 fault code means: your boiler has lost communication with its external controls or is stuck in an internal testing phase, meaning it cannot start the heating cycle.. It appears on 3 Vokera models in our database.
Technical description: Temporary fault code (Refer to section 7.10)
What causes the Vokera 21 fault?
This fault occurs when the boiler's main control board loses the electronic signal from the external timer or room thermostat, causing a break in communication. It can also happen if the boiler becomes stuck in an internal calibration mode after a power cut or a system reset. Essentially, the 'brain' of the boiler is waiting for a command to start that it can no longer hear.
DIY fix possible
Locate the selector switch on the front panelTurn the dial to the 'OFF/RESET' position and wait for 10 secondsTurn the dial back to the 'Hot Water' or 'Heating and Hot Water' position
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vokera models that show the 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vokera 21 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