Need help with 2005 GP GXP
 

Hi guys! I need some help on a major issue I'm having with my 2005 Grand Prix GXP 5.3L. Let me start off with a bit about myself. 20+ year state certified master mechanic. Ive been around the block more than a few times. This is my personal car and right now, I'm at a loss.

Lets go back about two years ago. One day, I suddenly lost my cruise control, tap shift and my compass now says "CAL". I needed to upgrade the TCM and PCM anyways so I had a custom tuning company flash me performance modules for the car, in hopes it would take care of those issues. I was wrong but the problems weren't that big of a deal. Lets fast forward to last weekend. I was driving along during a country drive and suddenly I noticed my air conditioning went warm. I thought "oh great, just what I need in this heat we have been having". I was less than 1/4 mile from my destination so I just wrote the AC issue off for that moment in time. A couple hours later, I was ready to head home and I remote started my car. It fired for about 5 seconds and shut off. I thought that was odd, its never done that before. I got into the car, and it started with the key. I started to head down the road and it was shifting VERY firmly and my CEL was on. Great, now the trans is in limp mode because of a stored or active engine code. I get it home and put it on the code scanner and it only shows a P0102 ( Mass or Volume Air Flow Circuit Low Input). I cleaned the MAF with MAF cleaner, reset the code and tried it again. Same results with a CEL and trans in limp mode. I put on my AC manifold gauges and the pressures were right where they are supposed to be to allow the activation of the AC clutch, but it wasnt activating. I thought maybe my MAF really took a dump and went and picked up a new one. Installed it and disconnected the battery for a couple hours. Hooked the battery back up and took it for a ride. Still no AC, trans still in limp mode and eventually the P0102 code was triggered.

I checked over most engine electrical connections, nothing visually obvious wrong. I'm throwing this out here in hopes to pick someone's brain for ideas on what the heck could be going on with this car.

Thanks in advance for your help!

would think its possible you have an A/C high pressure switch fault reporting false A/C system pressure to the PCM.
this will cause the A/C clutch to not engage because it thinks the system pressure is too high to allow more gas flow and resultant pressure.
the A/C system pressure reading false reporting much higher pressures back to the PCM will cause far more dramatic torque management influence then it would ever see during normal operation. this MAY be causing the MAF to report less than the minimum accepted value within the fail count pre-requistites

What does the BCM do in these cars?

Do you have a K+N intake system on the car?

Maf
 

I would assume your mass air flow sensor went out and that could be causing some of the other issues as well. I would replace the maf as Im fairly sure thats one of your problems. If youre into performance at all you can get an LS7 card style maf while youre at it instead of replacing the stock one. You will however need to tune for the new maf.. As for the AC I have no idea unless limp mode is causing it. I know when my car gets too warm or the battery is low it will cut the AC off. Could also be the pressure switch as stated above.