Any update on what fixed your issue. I have the same problem.

 

Sorry no answers. Just sent mine to shop because after having similar problems and replacing the 2 speed sensors? They might actually be input output sensor. Changed tranny fluid 3x and added additive, seafoam I think. Wouldn't go into 1st so I started it out in second then shifted back to D and it worked great for a day. Then it stopped completely. No reverse now. Nothing. I'll try and update you once the shop gets back to me.
C

 

I haven’t read or posted to the site in a number of years but I'm pretty sure I read in the past that the problem is the transmission fluid temp sensor or it’s wiring harness, which would requiring draining the fluid and removing the transmission pan to get to them.

I still have my 2016 Elantra, but it has low miles (21k miles as a disabled retiree), so my situation is not as urgent. That said, I too am experiencing minor transmission issues, with odd shifting as you guys described. Not too long after I bought mine I had one episode on a very hot desert day where the transmission shifTed scarily hard out of park, though I can’t remember if it was into d or r. Never did it again. Through all of its 21k miles though it will also shift strangely when the a/c is off, as if it’s a manual shift car, where you get that drag of manual clutch shifting when downshifting. It hasn’t been a problem because I alway have my a/c on. Recently though I started getting it in a more minor way even with the a/c on.

I’m probably going to do the fix myself, even though it is still under warranty, because every car I have owned in my long life has been damaged by mechanics. Every single visit. My only concern is the nonsense of modern cars requiring expensive scan tools and knowledge to reset the relevant computer parameters, which I believe is necessary after such a repair.

 

You can get a rebuilt unit from a Hyundai dealer for $800. Just need to install. It's easy to instal. I did my 2016. Took 4 hours in back yard. I did use an engine crane to lift tranny.

 

Hyundai Re-Man A/T is usually around $1700-1800, add atf, and coolant (if needed)

Pick up at dealer when it arrives, no freight costs, though you will need to return your core, otherwise it $1000 added core charge, and be sure you drain oil out of it too

There is worksheet for return of core, dealer parts dept will help fill it out with you