99 Grand Prix what do you do when your car won't shift into 3rd or sometimes even 2nd on your auto tranny Is it time to replace and what would it take to convert to manual?

If your car revs up without shifting and misfires, it could be a clogged catalytic converter. If the computer senses too much back-pressure it won't shift. Since the Clean Air Act requires an 8 yr / 80,000 mi warranty on cats, I'd recommend having this check out before you spend money on the transmission.