@jimborasco .....Stuck in 3rd, same transmission error codes, no matter which transmission is in it and it does it as soon as the thing is started, before ever putting it in gear.
"Stuck in 3rd, same transmission error codes, no matter which transmission is in it and it does it as soon as the thing is started, before ever putting it in gear."
Here's a thought which just struck home. Was the Transmission Range Sensor from the original transmission used on ALL of the transmissions? If it is broken or faulty, it could cause the transmission to go into 3rd. gear immediately upon starting the engine. Rarely is a rebuilt transmission supplied with a new TRS; the old one is invariably used. I would think it might throw a code if faulty, but then again maybe not.
The TRS moves with the gearshift selector to electronically "tell" the PCM (computer) which gear or position, Park, Neutral, is being chosen. It is nothing more than a multiple position electrical switch. If it is broken and stuck in 3rd. gear position, ya got it!
Accessed on the outside of the transmission, easily removed and replaced, around fifty bucks or so. Looks like this:
Check it out before delving into wiring. Just a thought. imp