To test the wires, you’ll need a multimeter. First, set the multimeter to the “resistance” setting. Then, touch one of the leads to one of the thermostat wires and touch the other lead to the metal part of the thermostat. If the multimeter beeps or shows a reading of zero, then the wire is good.
If the multimeter doesn’t beep or show a reading of zero, then the wire is bad and needs to be replaced.