Wont Shift Out Of 3rd

[stev0]

I have a 2003 sierra z 71 I believe the transmission is the 4l60E. It starts in third and will not shift at all! Any suggestions. 156k miles. Tran Oil is a little dark but no odor. I have the typical transfer case holes and will be replacing transfer case next week. The transfer case is the NP 261 I believe. Will a new transfer case fix the shifting problem too? I tried to get codes but will not give me any even though the engine light is on. I take it something is wrong with that too! Any suggestions would be greatly appreciated. Im not a super mechanical guy but a friend gave me his motor all data password so I will be able to look up anything you say and get all the proper diagrams. Also if any of you kind people that help me need me to look anything up on the all data let me know. Thanks in advance.

Steve

[brims]

Check your fuse for the cigarette lighter outlet, then you should be able to pull codes. That provides power to the OBDII port.