So bought a 2000 Silverado Z71, when I first got the truck it was not shifiting properly. When I would floor it would try to drop it to second, until i let out, there was no slipping while this happened, and the previous owner had the trans rebuilt. So i took it to the dealership and they said it had high line pressure, which could be due in part to plugged cat's or it could have a shift kit in it . So got rid of the cats tody and haven't taken it back yet but today, on the highway the speedo started freaking out, the abs and brake light came on, then the trans felt like it was trying to jump out of overdrive. Talked to a trans guy and he said possibly could be the speed sensor, but not sure. Has anyone else had this malfunction, or any suggestions to as what it may be?

 

A faulty speed sensor can cause numerous transmission issues. I would suggest getting that fixed and go from there. I have seen a case just like yours when a cat was plugged. The only way the vehicle would shift out of 2nd was to lift and then the trans would shift. Sounds like you are heading in the right direction. Vince