P0785 code popped up on my 99 Vert with a 4L60E Auto transmission. I purchased the 3-2 shift Solenoid and also replaced the 1-2 shift Solenoid and the 2-3 Solenoid. Drove great for 50 miles after the shop replaced them, but the code just popped up again and it shifts terribly, and seems to not have 1st gear when the check engine light turns on. I can turn off the code and it will shift normally, but will pop up again shortly thereafter.

Another shop told me that these C5's have so many issues that are related to a bad ignition switch. I told my tech Tommy, and he said that he was researching that. I pulled the old ignition switch, and the most of the contacts were toast. Looks like a bunch of sets of points.
Anyway, replaced it with a new OEM switch, and the same issues happened, albeit much more often now (probably a coincidence).
I drove it 50 feet and check engine came on with P0785. The car is still pretty cold. I cleared the code and it immediately came back without driving, just idling. We went through this nonsense about 7 times. I went to turn around to drive it back into my driveway, and the check engine light stayed off. I drove it for a while and it performed as normal without any check engine light. I'm now wondering if the problem is just when the transmission is cold. Each time I have an issue, it goes away after I clear the codes several times while it warms up.
Oh well... I have a new ignition switch, 1-2. 2-3, and 3-2 solenoid (twice), new transmission oil and I'm out $600 bucks and still have the issue.

What to do next?? I have no other codes

 

Check the wiring to the solenoid, that code is a short to ground.