Set Appliance State:
Set Multimeter to Continuity 
Isolate Mains Power to appliance as per TOPs P4
Appliance On/Off control set to Off position
Remove the green wire from the flame sense electrode
Remove connector ST16 from the PCB
Appliance On/Off control set to Off position
Remove the green wire from the flame sense electrode
Remove connector ST16 from the PCB
Measurement:
Measure the continuity of the flame sense electrode lead.
Reconnect both ends of the lead after test.
Reconnect both ends of the lead after test.
Isolate as per TOPs P4
and refit as necessary