1998 Oldsmobile Aurora
My check engine light is on and the code is for the overdrive clutch stuck in the lockout position. Of course my miles per gallon has dropped off as a result. Is this a simple selenoid replacement outside the transmission or is it a teardown/overhaul? Thanks in advance for any help. cpojm

What is the actual code? I'm going to guess it is a P0741. If that's the case, the answer to your question is it will require teardown.
The transmission needs to be removed from the vehicle and the TCC solenoid and some updated seals will need replaced.