Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Continuity Set meter to measure Continuity
Remove the electrode assembly
Note:
Due to the electrode gasket being disturbed during this inspection, a new electrode assembly must be fitted even if it is deemed to be satisfactory

Measurement:

Inspect flame sense electrode.
Thoroughly check for damaged or cracked insulation and that the tip is not damaged or distorted.
Check for continuity at either sides of the sensing electrode.