Set Appliance State:

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

Measurement:

Check for continuity (zero resistance) between the
inline connector to the flame sense electrode to pin 11
(white wire) on connector ST16.
Visually inspect that the harness has no signs of water ingress and is in good condition throughout.