Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the flame sense lead (blue lead) from the flame sense electrode
Disconnect connector plug marked ST16 from PCB

Measurement:

Check for continuity (zero resistance) between
the ends of the flame sense lead.
Check that there is open circuit (O.L) between
the flame sense lead and the boiler casing.
Inspect the harness and connectors
for good overall condition.