I have a 2000 Pontiac Montana with a 4t65e tranny. It will not upshift into third gear. I can force it to downshift to third but it slips horribly bad and just goes to second as soon as you apply any throttle. I have replaced the fluid and filter and inspected the 1-2 and 2-3 accumulator pistons. I know that the pcm is commanding the third shift, but I have no gear ratio change. What is wrong with this thing? HELP!!!!!

Sounds to me like you have badly worn clutches.;.But im no trans expert.