Fluid cannot be over the FULL mark when Hot...otherwise it can cause aeration issues and loss of pressure....
Have truck on level ground, apply handbrake, start engine and allow truck to warm up...have a helper put the brake on, put the truck in drive and then check the fluid level...it should not be over the FULL mark...
Does the fluid smell burnt?
Slippage errors like P0894 pop up when output shaft rotational speed against engine crankshaft rotational speed is outside of a predetermined range...
It will also popup if the lockup convertor clutch is failing....
These transmissions are usually pretty strong, so long as they have been updated with better pressure control systems, clutch packs and band servos....the rate of attrition on the rebuilds tells me that perhaps you might be better served by a different shop.
Re you performing the R&R on your own, on the ground...I have been a GM tech for over 40 years and there is no way I would attempt one of these nowadays...back when I was young a foolish...maybe...but with the trans unit alone weighing some 280 pounds, plus the 4WD unit if equipped...
4 post hoist, trans jack, nice level concrete floor...is the way to go...
Hope my thoughts help you out
