Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X5 from the main PCB
Remove connector from the flue sensor

Measurement:

Measure the resistance across each wire of the flue sensor harness, pins 8 & 13 (violet wires) of connector X5 to the flue sensor connector.
Expected resistance continuity / zero resistance on each wire.
Note.
Ensure a good connection is made or misdiagnosis will occur.