Set Appliance State:

Mains Power Off
Turn Hot Water Tap ON
DHW Temperature Control to MAX
Set flow rate to approx. 3 litres per minute
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Preheat switched OFF (ECO button illuminated) A.S.A.P. after power reinstated.
Does the fan run?