Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the harness from the Return sensor

Measurement:

Measure for continuity (zero resistance) of the two blue wires of the return sensor harness (pins 12 and 13 on X22 at the PCB).
Inspect the harness for good overall condition.