Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X51 from the PCB
Remove PCB to interface PCB connector

Measurement:

Measure for resistance across each wire of the interface harness.
Expect continuity / zero resistance on each wire.
Also visually inspect the harness for good general condition.
Are the resistances as expected and is the harness in good condition?
Note.
Ensure a good connection is made or misdiagnosis will occur.