[Captain J]

Originally Posted by Dead Parrot

One possible thing that might cause a shifting issue is if the o-ring on the filter tube is damaged, worn, or missing.
Amazon link that shows the o-ring as part of a kit: https://www.amazon.com/ACDelco-TF231...2935891&sr=8-3

I noticed that some kits don't show the o-ring. It is needed or you won't get good fluid pickup and will have symptoms similar to low fluid levels. Including bad shifting.

#7 on the diagram shows where it goes. Another thing some folks miss is removing the old one if it sticks in the opening when you pull out the tube #14.

I would finish your check of the modulator and related stuff before pulling the pan. Pulling the pan probably means replacing the pan gasket + 4~7 quarts of ATF. No point in doing that until you know the problem isn't in the modulator you already have off.

You are correct,
This is not the issue, otherwise shifting problems will happen after the oil change.