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 connector X9 from the PCB
Disconnect the in-line connector which connects the flame sense electrode to the flame sense harness

Measurement:

Measure the resistance of the flame sense harness from the grey wire at connector X9 to the opposite end at the disconnected in line connector.