03 sierra z71 stock

100 miles ago i had a new t-case and tranny installed at a gm dealer. yesterday morning i hop in and start the truck and its in 4 low. i never put it in 4 low the night before. later that day again i hop in and its in auto 4x. same thing happened again last night, it was in 4 high when i started it and later it started flashing neutral in red. this morning the service 4wd comes on. obviously i have to take it back, but im wondering wtf is going on?

100 miles ago i was leaving a parking space, when i put the truck in reverse nothing happened. truck wanted to back up, you could feel it pull just a bit, then wham wham wham the truck jerks backwards hard, then nothing. no gears at all. dealer said the t-case took a dump so they installed a new one. on the test drive, the tech said the tranny was making some funnt noises. gm told the dealer to just put a new one in (still under warranty). so thats the back story.

 

The "Service 4WD" message comes up when there is a problem with commanded signal and actual signal. The most common cause is a failed encoder motor sensor. But there are many other items that could cause the message. It's not, as some would like to think, a reminder to service it. Last time mine failed it was the encoder sensor, did the same thing you described. I even tried to drive it and it drove like **** because the trans was trying to shift on 4LO tables when it was actually in 2HI. Most likely a $75 part but it will end up costing many times more at the dealership. Good luck.