Set Appliance State:

Mains Power Off
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the fan to run and observe the spark electrode through the sight glass.
Can the electrode be seen sparking?