Why wont my 2001 gmc sierra 2500hd go into 4th gear?

Wiring diagrams.

https://www.google.com/search?q=2001+gmc+2500hd+headlights+wiring+diagram

and

https://www.google.com/search?q=2001+gmc+2500hd+headlights+wiring+pinout

Plus

https://www.alldata.com/diy-us/en

..

check the gear linkages may be loose.

You most likely wont be able to due to the antitheft system. This link explains the system.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

02 gmc sierra denali making a vibration noise about at 50-60 mph slower speeds no noise.please help

on the left rear of the engine next to the last ignition coil, connect to a hose that goes up to the rear of throttle body/ upper intake

safety[ limp] mode truck wont go over 2000 rmp but it will get u safely off road or to a service station or home ur getting some kind of code thats effecting ur truck --use a different reader urs might not be able to retrieve the problem code but can reset the computer-some auto stores such as auto zone / advanced auto scan for free check shops in ur local area

not sure here but i would hazard a guess that the operating solonoid is weak

pull thr speedo sensor out and look at the end, it may have been damaged by dirt oe metal shavings in the transmission. let me know how it looks.

There is a speed/shift sensor you could change, but if you haven't changed the fuel filter in a while, i'd go there first. Sometimes, in fact, often, plugged fuel filters can cause the engine to starve for fuel, and make it seem like the transmission is the problem.

yes i have heard of this and my friend is on the second transmission in his 2006 2500HD.the first time there was internal issues and the second time they ended up changing out the whole tranny.