My "95 Seville shows “check engine” most of the time and how do you reset the outside temp reading?
You can take the Caddy to a major auto parts store like Autozone and have the codes read. That will give you an idea of what’s wrong. I think most of these temperature readouts can be nulled or reset by pushing 2 buttons together. Your owners manual should clarify this. Make sure to write down those code readings and give them to your local independent mechanic.