Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Carefully remove the electrode from the combustion box door
Carefully remove the electrode from the combustion box door
Measurement:
Measure the resistance between the tip of the sense electrode
and the lead connection point.
Also visually inspect for damage, cracked ceramic and
general good condition.
Expect continuity/ zero resistance.
Note:
1. The test points will differ depending upon which electrode you have, the red arrows indicate the correct test points.
2. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
and the lead connection point.
Also visually inspect for damage, cracked ceramic and
general good condition.
Expect continuity/ zero resistance.
Note:
1. The test points will differ depending upon which electrode you have, the red arrows indicate the correct test points.
2. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
Isolate as per TOPs P4
and refit as necessary