Woke up this morning and the downstairs Carrier TSTATCCPQ501 thermostat had nothing showing on the display. Replaced the batteries and display came back on but part of LCD was faded like it had gone bad. Pulled the thermostat control panel from upstairs which is the same model and put it on downstairs. The display comes on and shows 70c, am pm 18:69 day morning night etc, but I cannot change anything with by moving it to program set or with the up and down buttons. The only thing I can do is get the fan to turn on when moving it on On from Auto. The display just keeps showing the same thing.

I took the display off downstairs and brought it back upstairs now and it is now doing the same thing on the upstairs AC.

Please help. :(

Sounds like you may have a electrical problem downstairs, either with the wiring, or component that's shorting your thermostats, and should have it checked by an tech/electrician, unless you have an ohmmeter and know how to use it.

Even with batteries, there is a power supply on the control board. Look at that to see if anything is flashing.