I have a 2001 Bonneville and just the other day.

I have a 2001 Bonneville and just the other day it quit shifting into overdrive. I check the fluid and it appears to still be a nice red color, but it is way over full and has air bubbles in the fluid, but not frothy(milkshake). The car was running and on level ground when I check the fluid. It doesn't seem to matter if the car is warm or cold, it won't shift into OD (4th) either way.

Fuse is good. Still shifts Reverse thru 3rd smoothly.
I am going to pull the pan and replace the filter today but that doesn't fix it, I am not sure what to do next.