After 2 days of the Hyundai dealer mechanic scratching his head after getting no codes reading out, he concluded that the problem must be internal which would require removing and disassembling the transmission. I suggested he refer to the Hyundai service bulletin on how to check the transmission lever switch. Later in the day, I got a call, the switch was defective and I was charged $ 400.00 labor for my diagnosis. I love the car, but avoid dealers at all costs, in this case I could not find an independent shop that would take on this problem.

 

yes had this problem the only thing i had to do was pull the console up from the gear shift and clean it out the contact get either dirty and i have also spilled some soda down in there and it went crazy hanging in whatever gear it want to.Didnt go to the dealer because im out of the waranty. It has happen atlease 4 times. take the gear shif lever apart and clean the contact and go on with life. Ive got 200. 000 miles on this car already never been back to the dealer.

 

Jarmar20 - I recently had the exact problem you and peter073 describe in my 2006 sonata w/93K. Found this thread and did exactly as you described - removed the console and unplugged the connection from the gear shifter. Figured the whole time there was no way it would be that simple. Sure enough, there was definately some corrosion in there. i cleaned it good with some soft wire, added some dielectric grease to the contacts, and plugged it back in and BINGO. Works like a charm. Thanks for saving me an expensive trip to the dealer.

 

The contacts in the console of the sonata are very sensitive to dust water moisture u may try to go in clean the contacts in the gear shift lever console and put it back to work . I have spill coffee in it on my way to work 3times and it like driving a stick shift u have to move it in manuel mode to get from 1st to 5th.

 

My 2000 Hyundai Sonata GLS 6 cylinder began downshifting 1 gear when I would be driving at high speed, usually after about 20 minutes of being on the road. I changed out the output speed sensor and the car began downshifting sooner. I changed out the input speed sensor – problem solved! The input speed sensor cost about $80 + tax. Both sensors are under the air filter, the input sensor is under a cable support bracket. It takes less than an hour to do it. Peace & Blessings!