I bought a 2006 g6 gtp used with a 3.9L transmission 4t65e. The transmission was in limp mode do to code P0700. I changed 5 shift solenoids the code went away but after driving the car for a short while around a 1,000 miles the transmission started slipping mostly when cold. But then started making noise and then would not go at all if put it into drive or reverse. But if i shut the car off then started it back up it would go into dive and reverse. What could make this happen.

Many things.

How many miles on the car?

Has the filter been changed?

Is the fluid level full and not overfull?

Did you test drive it very much before purchasing?

Do you know any of the history of the car?

In what region is it located?

There is 186 thousand miles on th he car. I changed the transmission filter and fluids when i replaced the shift solenoids. The transmission fluid is not over full nor is it low. I put 7.5 quarts of transmission fluid in when i changed it. I live in Michigan. I was told by a local transmission mechanic that it could be the torque converter and pump is going out. Dont know why it slips after the car reaches normal operating temp. Or i will lose drive and reverse. But when i shut the car off then start it back up i have drive and reverse again. I can power break the car and it will not slip until i put the car in park then back into drive then it will slip or it won't work at all.

Sounds like something gummed up or leaking in the valve body to me.

This is a 4-speed automatic, right?

Beside valve body, could also be sticky accumulator bores.(where you can install the 4T65e Transgo shift kit) helped me to stop the slip or clunk! LOL..
What I would try is a good cleaner, & I use this before a pan drop & filter change, but is safe to use anytime.."SEAFOAM" Trans Tune.
If you can add a half to 3/4 of the can & drive it for a while & see if anything improves...may help IDK..
I use Dexron VI (6) fluid and if you decide to drop the pan again, you can clean up the 1-2 & 2-3 bores with fine 1000 grit emery cloth.
At 186K miles it may just be best to rebuild the 4T65-E and get it cleaned & inspected, were there any fine particles in trans pan?
Post # 43 shows pan to accumulator bores & Thanks to Dan I got the Transgo Shift kit in, stopped the hard shifts anyways.
I'm at 110k miles and still shifts well, but I also expect a rebuild to my trans one day too.
https://www.gmforum.com/buick-172/4t...-302566/page5/

When i bought the car it already had the code P0700 and the transmission was in limp mode. So i changed 5 shift solenoids and the filter and added new transmission fluid. When i started the car and checked for codes after i put the car back together there was no codes. The car shift fine for about 1,300 miles then it would slip a little when the car was cold. When the fluids reached normal temps it shift fine but that only lasted 1 week then it just got worse. Was told it could be the torque converter or the pump or both going bad. The transmission does make noise it comes and goes. Kinda sounds like a buzzing whining sound. Whats weird is when the transmission slips or won't move the car at all i can turn the car off then right back on and the transmission will shift.

Sounds like a bit of junk in there clogging a passage, possibly right before or right after the pump. I imagine fluid flow pushes or pulls it into a clogging position and then fluid flow keeps it there. Shut off the engine, fluid flow stops immediately and the junk can settle away from what is is clogging. This scenario, if accurate, tells me that gravity has a play in the game. Perhaps gravity and slow-enough flow allows it to stay down and high flow and/or bumps cause it to get picked up . . . ?

When you replaced solenoids, did you closely clean and inspect their mounting positions and the passages they are connected to? A piece of junk may have been loosened while changing solenoids and now it is haunting you.

I am assuming at this point that we are talking about a 4T65E with two shift solenoids, a TCC solenoid, an EPC solenoid, and a manifold switch, right?

(Five shift solenoids implies a possibility that this transmission has up to 32 gears. )

What the best way to remove the 4t65e transmission pull the subframe and take it out the bottom or remove both motor and transmission out the top.

What DTC'* are you getting. The P0700 is just a code for the ECM to turn on the check engine light. Go into trans control and look for specific trans codes. When you shut the car off then restart it the module "resets"until it see'* a failure again but should have stored a trans related DTC. That may lead you in a direction. Hope this helps