My mom has 2000 grand prix with broken a/c. It's blowing hot air. The compresser never kicks in or out of cycle although it's not locked up. Both a/c lines are warm and only service port that I can fit my gage is right under the windshield and pressure reads "0". I've tried to fill some 134a and it's not taking any. Checked fuses and relay and they checked out. Any ideas?

jump the pressure switch and add some 134a sometimes the switch needs to be jumped for the compressor to kick in.