Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disengage DHW control knob from PCB potentiometer
& fit on reverse side of PCB (see image)
Ensure No demands
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Turn the DHW control knob fully anti-clockwise then back
to a central position, this cancels the preheat function.
Note:
Preheat is not a function of this boiler, however due to the PCB being used in various other boilers it is possible to enable preheat if the DHW temperature control is mis-aligned with the PCB potentiometer.