Set Appliance State:

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

Measurement:

Measure the resistance between the connector for
the flame sense electrode to pin 10 at connector ST16.
Resistance should be short circuit (zero resistance).
Inspect the harness and connectors
for good overall condition.
Note:
Colour of flame sense lead may change
but test point will remain the same.