1996 chevy z71 transmission in limp home mode

I have a 1996 chevy z71 truck with a 4l60e transmission the transmission is in limp home mode it seems to work fine when i first drive it but when it warms up it takes off in 3rd gear i check the fluid fuses under the hood hook a scaner to it no transmission codes i can manuly shift throught all the gears with the scaner and they work fine i check the wireing to the trans and it looks good and the wireing going to the pcm i just put a used pcm from the junk yard i bought this truck a week ago and it didnt run when i bought it and now i got it running good and the trans is in limp mode someone please help me thanks

I have a 1996 chevy z71 truck with a 4l60e transmission the transmission is in limp home mode it seems to work fine when i first drive it but when it warms up it takes off in 3rd gear i check the fluid fuses under the hood hook a scaner to it no transmission codes i can manuly shift throught all the gears with the scaner and they work fine i check the wireing to the trans and it looks good and the wireing going to the pcm i just put a used pcm from the junk yard i bought this truck a week ago and it didnt run when i bought it and now i got it running good and the trans is in limp mode someone please help me thanks

Was the transmission acting up before you changed VCM/PCM?

To check out transmission or get transmission codes you need a engine/transmission capable scanner.

OBD11 sscanners or code readers will not do transmission problems.

Post back any codes you get from a transmission capable scanner.

I m not sure if the tranny was acting up before i changed the pcm i bought the truck not running with a bad fuel pump and the guy i bought it from was trying to fix the truck and put a pcm in the truck witch was the wrong pcm he put a v6 4.3 pcm in a v8 5.7 and the truck wouldnt run right so i changed the fuel pump and the the pcm now it runs great the scanner i have is a red snap-on scanner that reads transmissions and engine codes and does lots of tests but no codes come up for the tranny i m not even sure my tranny is even in limp mode

I m not sure if the tranny was acting up before i changed the pcm i bought the truck not running with a bad fuel pump and the guy i bought it from was trying to fix the truck and put a pcm in the truck witch was the wrong pcm he put a v6 4.3 pcm in a v8 5.7 and the truck wouldnt run right so i changed the fuel pump and the the pcm now it runs great the scanner i have is a red snap-on scanner that reads transmissions and engine codes and does lots of tests but no codes come up for the tranny i m not even sure my tranny is even in limp mode

If it just acts up hot you may have pressure lose or slippage when hot.
But should set code or codes.
If you have the snap-on MT-2500 with 3 in 1 cartage or engine/transmission cartridge.
Read out transmission shift and slippage data and shift time.
Also make sure good fluid level and and fluid and filter.
Any stuff in bottom of transmission pan?