Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove flame detection electrode from its housing and disconnect from PCB
Remove flame detection electrode from its housing and disconnect from PCB
Measurement:
Test 1: Thoroughly check the electrode and lead for signs of damage, cracked ceramic and cleanliness.
Check resistance between tip of electrode and connector block at end of electrode lead.
Expect continuity / zero resistance.
Test 2: Thoroughly check the condition of the earth lead, checking for any damaged insulation.
Check the resistance across the earth lead from the PCB connector to the earth terminal connector.
Expect continuity / zero resistance.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
Check resistance between tip of electrode and connector block at end of electrode lead.
Expect continuity / zero resistance.
Test 2: Thoroughly check the condition of the earth lead, checking for any damaged insulation.
Check the resistance across the earth lead from the PCB connector to the earth terminal connector.
Expect continuity / zero resistance.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
Isolate as per TOPs P4
and refit as necessary