Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove fan and burner assembly

Measurement:

Inspect the flame detection electrode for any signs of warping, build up of debris or damage.
Measure the gap between the electrode and the burner.
It should be approximately 13mm.
Inspect the lead for good overall condition and carry out a resistance test between the tip of the electrode and the spade connector.
Resistance should be short circuit (zero resistance).
Notes:
Electrode shown removed for illustration only.
On refitting ensure all seals are inspected
and replaced if worn or damaged.