99 monte Carlo z34 trans problems

I just got a 99 Monte Carlo z34 with a 3.8 it has been running great for two weeks and last night I was getting on the freeway and I noticed the wasn't shifting into overdrive what could it be? I thought that it might b the tranny fluid so I checked it and it seemed old and dirty so I flushed it and changed the filter but that didn't fix it. What to do

try your passing gear cable on the throttle body.

I have found the same issue with an older car. It was the lock-up torque converter not engaging and causing a trouble code for No TCC engagement. When the code came in, it would come out of overdrive and would not allow overdrive to operate until the code is cleared. As soon as I would clear the code, it would shift to OD again. Found that the TCC Valve was stuck. Worked the valve loose and put the valve body back on. Never had another problem.

Again,, I know this is an older post, but I thought I would share what I have found so that a future user of this site would have the answer. Don't bother sending me a note about the age of this post.