Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove electrode cover (2x screws) then
inner combustion cover (5x screws)
Disconnect inline connector to flame detection lead

Measurement:

Check the detection electrode is in good condition and correctly positioned across the burner as in the picture.
Check that the gap between the burner
and the electrode is approximately 4mm.
Inspect the ceramic for any cracks or damage.
Check for continuity (zero resistance) between the
tip of the detection electrode to the connector and
that the lead is in good condition throughout.
Note:
Electrode shown removed for clarity
but can be left in situ for test.