I'm frustrated! I have 99 STS with 149,000 miles. No current codes and no history codes. 3 days ago, freon was 1.5# low. I went to my friend's shop. He used his machine and pulled the .5 out. Vacuumed etc. Put 2# back in (including .5 that was saved) and dye. Today it was all gone again. We used sniffer and black light and can't find any trace of leak. I would appreciate your help once again.