K3500 no codes, wont shift back into OD after kickdown to 3rd.

[xKid4x4s]

hey folks, Hoping to find some help diagnosing an issue here. Just bought a 2000 chevy K3500 Dually, 240K on it, super clean 454 runs great everything is in perfect condition but heres the kicker.

When Im driving it, at no specific time, warm, cold, long drive or short. When the transmission feels the need to kick down into 3rd from over drive it cannot go back into overdrive and runs in 3rd at high rpm.

There are no codes at all, have been told it could be an overdrive solenoid, the Throttle positioning sensor (doubt that one) or even the electronic linkage sensor?

any advice or help would be really appreciated, thanks have a good one!

[Warrens69GMC]

My boss had a similar issue and it ended up being the trans speed sensor. It happened on summer after doing a little towing , then the issue started.

[xKid4x4s]

Thanks, I thought about it being the trans speed sensor also, do you have to drop the pan for that? 4l80s are new to me. thanks again