Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Selector switch to DHW position
CH temperature selector to Min
DHW temperature selector to Max
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if it is in lockout.
Does the fan run at high speed for at least 15 seconds?