AL 21 fault code means there is an open circuit between the two black wires at connector X9.
These will have a wire link fitted between them on the additional external controls terminal strip (see image) from manufacturer, unless the appliance has been installed with either an underfloor heating limit thermostat or condensate pump limit switch - in which case these wires will lead to relevant control.
Investigate further and confirm correct operation of any underfloor heating controls or condensate pump where applicable or the integrity of the wire link if fitted.
Attempt to reset the appliance once operation of controls / satisfactory link has been confirmed. (Control knob to OFF position then back to DHW and CH)
If the appliance will not reset a new PCB will be required.
These will have a wire link fitted between them on the additional external controls terminal strip (see image) from manufacturer, unless the appliance has been installed with either an underfloor heating limit thermostat or condensate pump limit switch - in which case these wires will lead to relevant control.
Investigate further and confirm correct operation of any underfloor heating controls or condensate pump where applicable or the integrity of the wire link if fitted.
Attempt to reset the appliance once operation of controls / satisfactory link has been confirmed. (Control knob to OFF position then back to DHW and CH)
If the appliance will not reset a new PCB will be required.
