My ACs are the original 13.5 duo therm units. The rear unit runs as it should. The front unit runs fine on the heat pump (compressor runs), on AC the fan runs but the compressor doesn't ( does not try to start) any ideas? I hav checked all the phone cable connections as well as the connections at the control board up top. All dip switches on the front control board are off. As best I can tell that's where they should be.
Do you have 2 furnaces? If you do then the furnace dip switch should be on.
I do have 2 furnaces, no furnace dip sw 2 thru4 zones , stage and heat strip. Control board is 3106482.015.
All functions on both zones work as they should just no compressor on front ac in ac mode.
Do you have the original thermostat? It might have failed. If you can switch the wires in the thermostat and the problem goes to the rear then you would know.
I have a 4 button comfort control thermostat. Phone cord connection only. I'll have to see if I can identify the zone 2 connection at the front ac.
You suppose it's time for a new unit and thermostat? I'm in the same situation. Originals still working kinda sorta!
For those thinking it's the 4 button controller, consider pinging Jim Frerichs. His ad is old, but he's still active on the forum.
Dometic 4 button thermostat repair (https://www.foreforums.com/index.php?topic=21786.0)
Problem solved! In the process of removing the capacitors from the front Ac I noticed a 2 wire plug hanging loose. It is the remote temperature sensor so I plugged it Back in. Unplugged it this morning everything works as it should. Go figure.