I have a 1991 L98, with the C68 A/C control system option. Yesterday morning I went for a ride and all was well. Later in the afternoon, I was about to go out again and started the engine as usual then looked down to set the A/C controls..
That's when I noticed that the set temperature was at 15 degrees. I normally can't set it below 60 degrees. When that went out and switched over to indicate "Outside Temperature" it was showing 28 degrees. That struck me funny as it was close to 100 degrees outside. I can't seem to change anything by pushing buttons"
I just checked for codes and found none present. What the heck is going on??

 

Not all that unusual.

Hopefully its just loose or dirty circuit board contacts. The display board slides into the main board that has a big plug-in on the backside. As they age the contacts get loose or corroded. You can pull it apart and lay strips of vinyl tape on the opposite side of the contact tab so that it makes the tab thicker, which makes it tighter in the socket of the board that it connects to. Gently abrade the contact surfaces with 600 sand paper or a nail file just to scuff the surface slightly. When the boards fit together tight, this often corrects the weird numbers and sometimes greek symbols that appear.
If there are functions but the display numbers are AFU, then the outside temp sensor might be bad. usually that will limit itself to that particular function. There is an inside sensor as well.

Worst case is having to get the control head rebuilt....thats around $150 exchange. I know most of the catalog Vette parts places offer that service. Some local vette shops in your town might also offer the service. The control head gets sent out to one of only a few places in the country that rebuild.. Same type of service for digital dash, ECMs, etc. Usually takes 1 week to 10 days turn around time.

Try the circuit board contacts first, and if there is no improvement, and the basics (fuses, wiring) are ok then start looking for someone to do a rebuild. I did mine last yr and what I got back was like new. New face plate, clean, like new.

 

Not sure if looks like the 80's unit, but you may still find this usefull.
http://forums.corvetteforum.com/1568603057-post3.html

 

It sounds like you are indicating centigrade rather than Fahrenheit. Did you perhaps push the MET/ENG button to switch it over? (Not sure if the 91 has that button).