Set Appliance State:

Mains Power Off
Gas Off
Turn Hot Water Tap On
Selector switch to DHW position
CH temperature selector to minimum
DHW Preheat selector to OFF position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary.
Does the fan run at high speed for at least 15 seconds?