Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove ionisation lead from CN17 / FLAME on PCB
Disconnect ionisation lead at the inline connector

Measurement:

Measure the resistance of the ionisation lead.
Expect continuity / zero resistance.
Also visually inspect the general condition of
the lead and for any signs of damaged insulation.