Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the inline connector on the detection lead

Measurement:

Test 1:
Inspect the detection electrode for good condition, cleanliness, damaged ceramic, correct alignment as per the fig 1. and that the gap from tip of electrode to the burner is correct.
Test 2:
Measure the resistance from the tip of the detection electrode to the pin in the inline connector.
Expect continuity / zero resistance.
Is the electrode satisfactory and the resistance as expected?
Note:
This test is carried out with the electrode in situ but is shown removed for clarity.