o/d light flashing

i have a 1995 ford probe, its a dohc 4 cylnder, the o/d light is flashing and when i put car in reverse it doesnt move and when its in drive it goes kinda but only 1 gear anyone know how i can check the codes?? also i just recently did tune up and i put trans-x in the tranny to clean it out because the old fluid smell burnt...

any advise? or do i just need a new tranny??
also how do i check codes