Usually when the temp symbol flashes a code is set in the HVAC control head......this can inhibit the functions of the controller....

On some units, you can access the codes by pressing combinations of buttons on the control head......i.e., I think pressing the OFF and temp UP buttons at the same time, puts it in diagnostic mode.....can't remember what button you press next to get the 2 digit codes....

If you manually can't get to the codes, then you need a high end scan tool, to read the codes....

Do you have a scanner? My 97 Olds 88 had a vacuum leak which caused my fuel trims to go out of whack. My A/C would only blow through Defrost. I had a crumbling vacuum hose under the cowl cover in the engine compartment. Remove the plastic cover that hides the Maxi Fuses and check for leaks.

I had this problem on a '97 LSS a few years ago and it turned out the vacuum line manifold had deteriorated and some of the nipples had collapsed, which is very common on H-bodies of that era, especially the ones with electronic thermostats. The solution was to eliminate the manifold by getting a couple feet of silicone vacuum line, slightly larger than the car'* climate control vacuum lines, and cut it into two-inch sections. The car'* vacuum lines are stiff enough to push into the larger line, so just push like colors into each end of a piece of the larger line as far as you can get it to go. You'll need to remove the glove box to get to the manifold. Once I made this fix, I never had another problem with it.

Same on my 92 88. Vacuum tank was leaking. Some RTV sealed it up and everything works good.