Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the electrode assembly from the heat exchanger

Measurement:

Visually inspect the condition of the electrode for any signs of damage, cracked ceramic or warping.
Also measure the resistance across the electrode (it may be necessary to clean the electrode first).
Expect continuity / zero resistance.
Is the electrode in good condition and the resistance
measured as expected?