One problem related to transmission sensor has been reported for the 1999 Hyundai Elantra. The most recently reported issues are listed below.
The transmission would jump out of gear or down shift and the check engine light illuminated afterwards while driving after the vehicle was parked overnight. The check engine light remained on. Basically this only occurred in the morning. Sometimes however, it would feel as though it was not shifting properly. Has not been taken to dealer. This started recently. Took vehicle to an auto parts store to check diagnostically, it was noted there was a problem with manufactured control module, input/output speed sensor, and the transmission range speed sensor. No repairs were made or corrections to this vehicle done at this time. Dealer's information was unknown by consume at the time of this complaint.