Fixed a '06 Impala driver's seat. Only 90k miles. Thing was slammed all the way to the floorboards, and wouldn't move. It went back and forth, but wouldn't move up and down. Could hear & see the motors trying to move the seat, but they couldn't for some reason. Found out why once I got the seat yanked and dove in.
Seems whatever limiting device that's supposed to be on there failed, and caused both drive screws to bind up solid. Had to really put some muscle into a pair of vice grips to get the SOBs to move!
Worked on it in the back of my truck - had the front of the seat down on the tailgate, and the seat back resting on my tonneau cover, so I could work underneath it. See 1st pic.
Had to remove the drive cables (that was fun ... and getting them back in was even ... "funner"), & take apart the plastic gear housing, & remove the worm gear in order to relieve the tension, and be able to wrench the drive screws backward.
While I had it apart, I threw some J/D Corn Head grease in there - can't hurt. Threw some all over the drive screws too, although it's probably just going to collect sand and grit.
Was a complete pain in the ass. No surprise there. Can see in the pic how much room I had to get my 1/4" wrench in - could barely fit a finger in there. GM builds this stuff to be totally unserviceable. To that, GM can guess which finger I'm extending.
Maybe this will help someone out in the future, when their seats seize up solid too. I'm sure this applies to all their models in one variant or another.
Just FYI - there's a little piece that looks exactly like a watch battery that will fall out of there when you disassemble that little plastic gearbox. It goes right on top of, or under (depending on what side you're working on - can change orientation) the driven gear (the drive gear is the worm, the driven is attached to the drive screw). I don't have a good pic of it here, but you might be able to see it in some of them:
EDIT: Found a pic of the "watch battery" spacer I was talking about.