Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Isolate DHW cold feed
Selector switch set to DHW/CH
Preheat switched OFF
DHW temperature set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Is D still displayed?
Note.
Isolation valve is fully closed when in the horizontal position.