atc unit?

97 concord how do i know its the head unit? is there a way to test it?
or is there a trouble code. if so does anyone know the code can i access the codes by flashing the ignition key like cars past.

There is a diagnostic feature on the ATC you can try, not sure if it will help but it's worth a shot.

1) Start the vehicle and turn the Temperature control to the 75 degree reading on the display.

2) Simultaneously press and hold the Floor, Blend, and Defrost buttons (Top row of buttons, first 2 from the left and the last button on the right) until the display blinks then release.

3) the display will continue blinking while the system performs a series of tests for about a minute then will show a 2 digit code between 23 and 36 on the display if there are any codes present, or will stop flashing and show 75 degrees if no codes are found. If the display shows a code write it down then press the Panel Button (bottom left button) to see if more codes are present, it will only show 1 code at a time so continue pressing the Panel button until all codes are shown. Code descriptions are below.

23 - ATC blend door feedback control
24 - ATC mode door feedback control
25 - Ambient temperature sensor
26 - ATC in-car sensor
27 - Sun sensor failure
31 - ATC recirculation door stall failure
32 - ATC blend door stall failure
33 - ATC Mode door stall failure
34 - Engine Temperature message not received
35 - Evaporator temperature sensor failure
36 - ATC head communication failure

Don't know if there are any 28-30 codes, if there are my book (Haynes) doesn't show them. Good luck, hope that helps.