Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove connector to DHW flow sensor and place in a safe place
Selector switch set to DHW/CH
DHW temperature set to MAX
Preheat switched OFF
Ensure NO demands are present
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Is D still displayed ?