Exterior temp on A/C controls

I have a 1999 Grand Prix GT. I am having a problem with my a/c not blowing cold air when the exterior temp readout on the a/c controls reads low. It has been messing up lately and not displaying the correct ext temp. For instance, yesterday it was 90+ degrees outside and the ext temp on the a/c system said it was only 22 degrees, so it would not blow cold air. That was fun! Any ideas?