904 Transmission Stopped Going Into Gear

TrailBeast said
Every single time a trans needs the throttle played with for the car to move it needs a rebuild.
(Sometimes) a full bottle of stop leak or "conditioner" can make the seals softer and they work for awhile, but it's a total temporary fix.

I have NEVER seen one come back from that by adjusting or changing any part except a really plugged up filter, and if the filter gets that plugged up it's coming apart inside anyway.

Sorry for the bad news, but that's just the way it is.
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That's pretty much what I was afraid of.

I think I might pull the pan, look around, and replace the filter, add some "conditioner" and see what happens. If the trans is shot, what can it hurt?

If it needs the full rebuild, is this something a mechanically minded person can do at home? I know there are tons of resources and guides online, I would just like to hear from someone who has actually done it.