Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Disconnect inline connector for flame detection electrode

Measurement:

Check for continuity (zero resistance) between the disconnected lead to the black wire at CN104 on the ignition PCB.
Inspect the harness and connectors
for good overall condition.