One problem related to ignition has been reported for the 2004 Hyundai Sonata. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Hyundai Sonata based on all problems reported for the 2004 Sonata.
Emergency flashers on my 2004 Hyundai Sonata began flashing for no apparent reason and could not be turned off (ignition was off), this continued for 15-30 minutes from the time it was noticed (neighbors knocked at the door to report that the flashers had been left on). The occurrence repeated several times throughout the night and the next day. Accessing the vehicle using the keyless entry, the doors re-locked themselves, and this repeated each time I unlocked them. The flashers stopped when the door was open or the car was turned on, but when the door was opened there was a clicking noise in the dash (at the same rate as the flashers had been flashing). We took it to the dealer for repair and they said it is the emergency flasher 4-way switch (part #937903c000) -- which is on national back-order for one month! this part is apparently not covered on the gold plus protection plan!.