Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the detection lead from the ignition control box

Measurement:

Test 1:
Inspect the detection electrode for good condition, cleanliness, damaged ceramic and check the condition of the lead.
Check the gap from the burner is 6.5mm +/-1mm.
Test 2:
Measure the resistance from the tip of the detection electrode to the ignition control box connector.
Expect continuity / zero resistance.
Is the electrode/lead satisfactory and the resistance as expected?
Note:
This test is carried out with the electrode in situ but is shown removed for clarity.