I praise Dodge for adding diagnostic codes to the ATC system. It's a big complicated mess with many sensors, servos, doors etc for the vent, defrost and ac all to work together in harmony.

Problem is rarely does that stuff fail. It's most often the ATC head unit itself. And guess what. There is no diagnotic code for an internal failure. Only codes to blame something outside the unit.

My 94 LHS defaulted to high fan, high heat, defrost. Found a used replacement. It failed the same a few years later. Opened up my original and found a crack line of solder on the circuit board.

Resolder both units and they both work again.