I had a problem with the ATC on my Concorde. The display and heat was working intermitently over the past few years. Sometimes the heat would blast, sometimes no heat. Other times the display would show and next time I started the car nothing. I purchased a Heating and A/C Control Unit on Ebay for $20. This is the display with buttons in the dash. Just snap out the plastic trim and remove three screws. The thing connects with two plugs. As soon as I started the car the display came on and I could control the blower, A/C, etc. The problem is the heat was blasting. I figured it was the blend door. On another website I found this:

Run ATC Diagnostic:

Set Temperature to 75 Degrees. Press & hold the Floor, Mix & Defrost buttons untill the display blinks.. release the buttons. Pressing and releasing panel button will display next code.

Codes:
23 ATC blend door feedback control
24 ATC mode door feedback failure
25 Ambient Temp sensor
26 ATC in car sensor
27 Sun sensor failure
31 ATC recirc door stall failure
32 ATC blend door stall failure
33 ATC mode door stall failure
34 Engine Temp messege not recieved
35 Evaporator temp sensor failure
36 ATC head communication failure

No error codes came up for me but it seemed to reset the BCM computer. Now it works great. So simple yet took a while to figure out. Thanks Google!