If you have normal acceleration from a dead stop, it'* in first......if sluggish from a dead stop, it'* second....

I have the official from GM shop manual, not the ones you get at a local parts shop but the big one used by the dealerships. Code 18. I followed the step by step procedure listed in the GM shop manual. Replaced cam sensor and crank sensor and wiring and still code 18. Come to find out it was a bad coil; nothing to do with a code 18 at all but that is what the computer reported. Later after replacing the coil I got a code 19. Same thing as before, follow the manual; forget all it said could be but one of them was a bad computer. Well, turns out it was a bad spark wire; nothing related to a code 19 at all.

Yes, the codes can be helpful and many times it is the bad part that the code indicates. However, you have to know more than just code checking. This is where experience comes in and is where the mechanic in California knows. He writes all the time in his blog about people coming to him after going to someone else who just checked the codes. My sister has an incident where her garage kept changing out the O2 sensor because the code said it was the O2 sensor, it was not; yes, that is what the computer said code wise but it was not a bad O2 sensor causing the issue.

And sometimes a failure will not set a code; I have been there as well.

I would look at getting a scanner that captures data. I had one on loan for my car and it could store the data so you could retrieve it at a later date. You could look for a used one on ebay and Graigslist. Doing a stopped scan is not really going to tell you much about the sate of the shift solenoids.

wiring

It ended up being an internal failure.

Thanks for the update, did you wind up taking it to a transmission shop?