My 2000 Kia will not sift

well depending on the circumstances its hard to say whats going on with it, try having the fluid flushed that might help but kias are very fragile by doing a little when theyre young it will save you alot later some kias the transmissions go, but again people think you don't have to take care of them, i would definitely try a fluid and filter change before i said it was the trans, 80% trans gone 20% youll get lucky
the other thing everyone forgets is there is a shift lock button on all Kia's on the automatic shifter,make sure the " hold" light is not eluminated.The other thing is the transmission computer is under the dash just above the brake pedal, Gold colored looking box, make sure someone has not accidently half unplugged it.If you decide to work on transmission computer box,like remove it, disconnect negative side of battery, that little gold box is $ 1000.00 from Kia, $100.00 from junk yard.hope this gives you more info.