Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance

Measurement:

Inspect flame sense electrode lead for signs of damage to the insulation and that the connectors fit properly on their terminals. Use the multimeter to check the lead for continuity.
When refitting also check that the
lead fits securely to both the flame
sense electrode and at its
connection to the PCB.