01 Neon tranny problem

[amanaman1]

Got an 01 Neon 2.0 automatic 94K that won't shift out of second gear. Had fluid/filter changed 1K ago and worked fine for last 6 months, fluid level is good and clean.
It shifts from 1st to second ok and back to first upon slow down but won't go into third from second. Sometimes it will also jump out of second gear like it is going into neutral then it'll catch back into second. No MIL codes or pending codes. Not sure if this is limp mode or not but from my understanding limp mode only gets you second and nothing else plus mil code. I am not getting that. Engine revs fine and rpms will keep going as high as I let it when it is stuck in second gear. Someone please tell me this is a magical sensor problem or solenoid or something I can replace myself. I do most all my own repairs just never putzed with a transmission other than fluid/filter changes.