Set Appliance State:

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

Measurement:

Measure the resistance between each of the wires on the reed sensor harness to pins 11 & 12 (black) on connector CN31.
Expect continuity / zero resistance on each of the wires.
Also visually inspect the condition of the harness.