I would start with checking the fluid level and condition.
This is by far the leading cause of this issue. Low fluid level will cause all sorts of issues but the most common is non engagement or slipping.
If it were me, I would go ahead and service the unit and make sure that it is okay.
https://www.2carpros.com/articles/how-to-service-an-automatic-transmission
If the fluid is changed and it still does this then more then likely this is going to be a pump issue. So we would need to inspect the fluid for metal and debris.
If any is found then we need to check the line pressure next. I attached the info below on how to do this. If it is low then you will at least need a pump and I would put a converter in it at the same time.
However, no matter what it may be a good idea to rebuild the unit since it is coming out for this anyway.
Please let us know what you find or questions you have on this and we can go from there.
Thanks
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Friday, August 20th, 2021 AT 7:46 AM