David,
If you still have the "5 button dometic" thermostat try doing a reset. Turn the t-stat off at the bottom switch. Hold down the Mode and zone buttons at the same time. While depressed, turn the t-stat back on. If all is good, you will see "FF" in the display if not you will see "EE". This reboots the t-stat to all attached zones. Incidently, when you had the center air conditioner changed did the technician toggle the zone 2 dip switch to the on position so the t-stat can recognize it? Hope this helps, Fred