Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect X3 from the PCB

Measurement:

Check for continuity (zero resistance) between the ends of two yellow wires running from X3 to the wires marked "OUTDOOR TEMP SENSOR" at the terminal block.
Check that the connector and harness is in good overall condition.