P0756 is a diagnostic trouble code (DTC) for "Shift Solenoid B Performance or Stuck Off is the error I am getting:

I have had my hummer for a year now and it is a 2007 with 138000 miles. Last April / May time frame driving back from Dallas I lost forward gears, just got rpm's. Not knowing any better or what was going on I had finished driving back to Houston so when I got home I popped the hood and there was the strawberry milkshake in my radiator reservoir the Trans cooler in the radiator busted I guess and mixed the fluids. was barely in 1st gear. Well for my stupidity I ended up replacing the radiator, flushing the engine and replacing the transmission. Once I got it all back together in June 2018 it ran great, better than before no issues at all, and I checked the fluid on a daily basis. Then I drive to east Texas 2 hours away it was fine. Woke up with the family, got gas and started to head to a funeral and withing 10 minutes I felt the rpm go up but no drive power. I know what this feel is for. In disbelief I immediately pulled over and killed it. I popped the hood and there was the strawberry milk shake again.
UPDATE: I have replaced radiator, flushed motor, flushed transmission even though this time the mixture didn't reach it. I then took my brother's advice and ran 2 external trans cooler's to by bypass the radiator, It ran great for the 2 hour trip back home. I thought problem was solved, nope, Got up to go back to work, (Teacher/Coach), and take my kids to school which is a 40 minute drive and within 25 minutes was losing power. Added trans fluid and it was fine, then it wasn't. I got home with it acting semi normal. The next day I test drove it and I started off very slow, which I guess I didn't have 1st gear, then it drove fine through 2nd and 3rd gear but I had no Drive/OD and the engine light was on. Ran a scan and got p0756 dtc. I then dropped then pan again, replaced both shift solenoids, filter and fluid and it still does the same thing.
I am at a point of frustration, I am having to use my wife's car while she car pools to her job. I can't afford a trans shop because their are so many crooked ones where I live so I am hoping someone could help me out. thanks,

2007 Hummer H3

 

 

happened on my silverado replaced the shift solenoid and all was fine

 

Dropping the pan to clean it out, replace filter, gasket, add new dexVI, is what fixes this code. Sorta like passing a kidney stone. Worked for me @90k miles, still shifts like new. Only 20k since I flushed the trans, however the pan was never dropped. Preventative maintenance is the key.