Nest thermostat doesn't work on one unit.

[Legohair]

Working on a large building with 12 RTU's. They are all individually controlled by nest pro thermostats. All RTUs are identical. Have one unit that will not operate.  Checked thermostat wire continuity and voltage on both ends. Pulled new wire and nothing changed.  The thermostat software acts normal no fault codes. Turn on heat and no click from the micro relay. Says it's heating orange screen and all. Does not close contact. Take the wires off the unit. It closes continuity just fine. The unit is properly grounded and does not have any other visible problems.  Took one of the sister nests and base units and discovered problem persist. Like wise took the Nest from said unit and put it on other units. Works perfectly. Contacted nest help and got an hour long um fallowed by contact an authorized nest PRO.   Tried using a jumper wire situation at the unit and still have the same thing. Tried taking the brand new control board from a sister unit and installing on said unit and still it doesn't work. Hoping there is an electrical Engineer out there with an idea. Because Google has no idea. And we are at a loss ourselves. 

[CoolingWizard]

When you used the VOM multimeter, and check the continuity. At the RTU, remove the thermostat wires form the control board.  Wire the R wire to the C wire. The Y wire to the W wire.  Then go to the thermostat and measure the resistance of R and C and Y and W.  The resistance should be very low. If the resistance is high, the one of the wires is bad. Repeat this test with all wires in the cable. Is this how to verified the cable?

AC Cooling Wizard

[Legohair]

Test was done as described. My fluke multimeter had new batteries and the wire and a reading of less then 0.1 ohms. To spice up the story a Honeywell Home Pro digital  thermostat works perfectly fine on said unit.