Cadillac Repair: 2000 deville a/c, faulty sensor, compressor operation

Question
Hi, I'm having some problems with the a/c system. It started acting up last summer, the rear went out first and was fixed by the dealer. The problem is that now the car is out of warranty and there is completely no a/c. I took it to a mechanic, apperently there was no pressure. He put some in, but the a/c is still not working. Self diagnostic is showing a/c overpressure and a tempereture sensor out of range. The pressure can't be the problem, because it is no where near too much now. Can a fualty temp sensor make the computer think there's too much pressure? He also connected the compressor directly to the battery and everything ran fine. I'm going to try to take it to an a/c shop, the thing I'm afraid of is taking it to the dealer. The mechanic told me the computer needs to be programed? Does the dealer have something to program it a normal mechanic doesn't? I thought clearing the codes through the self diagnostic was enough.
Thanks for your time.  

Answer
Hi odw, With the a/c there needs to be a minumum of 1lb of freon in the system to allow the compressor to engage. Once the sensor tells the computor that the freon is low whether it is a faulty sensor or really has low freon in the system a code will store and must be cleared first otherwise the computor won't allow the compressor to engage. To the computor a faulty temp sensor can give a low reading and keep the compressor off even though the system is up to charge. A faulty high side temp sensor had me going for awhile so the sensors definitely need to be tested.

  If this were in my shop I would add 1lb of freon, clear the code(s) and see if the compressor would turn on. If the freon could be heard leaking out then of course that would have to be fixed first. Then leak test the system and test for compressor operation. Sometimes a complete charge has to be added until a leaks shows itself and adding a florescent dye with the freon would be good to do too. After doing that a recomendation as to what will fix the car would be given. Caddys seem to like to leak through the evaporator core so that must be tested also.

   Whether you take it to the dealer on not is up to you. The differance might be that the dealer has a Tech 2 scan tool that can do alot more that the generic scan tool that the independant shops usually have and theirs might not be able to read temp sensors or clear codes on your Caddy. Just make sure the shop is famillar working on your year Caddy or you might spend money training someone and hanging parts on your car that aren't needed. Hope that gives you some insight. Bill