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 CN5 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.