Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect flame sense lead from the electrode
Remove connector X2 from the PCB

Measurement:

Check for continuity (zero resistance) between the electrode end of the lead to pin 1 (black and white) at connector X2.
Check that the lead and connectors are in good condition and are able to fit securely onto the electrode / PCB.