Fail safe mode on 1g auto trannies

[MATCHBX]

Well, it hit me on the way over to my mother's house as to what's wrong with Terina's tranny. It won't shift up or down from 2nd gear. It works fine in reverse, neutral and park. I remembered reading a while back in the owner's manual that if the tranny detects a malfunction, it will go into safe mode. It will only have one forward gear (either 2nd or 3rd). The only thing they say to do is take it to the dealer to have them diagnose it.

My question is, is there any way to diagnose it without going to the dealer? Any codes to read or anything? I changed the tranny filter and oil tonite because the old stuff was brown. It had almost 25,000 miles on the last change. I also put some lubegard in it. I have a spare tranny sitting here but I really don't want to have to swap it out if there is an easier fix.