Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Disconnect flame sense lead (green lead) from
sense electrode and secure it as shown
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure voltage across the disconnected flame sense lead and a suitable earth point.
Expected voltage approximately 50 VAC