Where is my overdrive?

2001 Elantra,automatic.I stop on the highway and when I start back up I
notice the check engine light on.As I get up to speed I hear why it came
on,it won't shift into overdrive.The next time I stop, when I leave the
light is out and it shifts into overdrive.After the next stop,the light is
on and no overdrive.This is all happening on the turnpike with stops for
gas and coffee.At first I thought it was the coffee,but I geuss not.It
goes to the dealer on Monday,but are there any ideas what is happening?

 

I'm guessing there is a problem causing your transmission to shift into
failsafe (third gear only). Most frequently, this is caused by the input
speed sensor.

 

"Hyundaitech",your right again.I got it back from the dealer today and the
paper work statesutput speed sensor re,sensor asm,output speed sensor
add scan tool operation.In english this means it was broke and they fixed
it.I have to give kudos to my dealer who took it right in and fixed
it.From some of the stories I've read I feel pretty lucky to be getting
such excellent Hyundai service.A tip of the hat to Bowser oF Pittsburgh,PA

 

Well, I almost got it right. I said it would probably be the <i>input<i>
speed sensor. Glad to hear it's taken care of.

 

Hey DADEO how much did this fix end up costing you ?

Just curious as i think i may have a similar fault on my 2002 elantra.
Drops from overdrive but on next startup works ok.