Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Disconnect the lead from the flame sense electrode
(the rear electrode of the three electrodes)

Measurement:

Measure the resistance between the disconnected lead and the green wire (pin 1) on LV1.
Short circuit (zero resistance) expected.