I drive a hyundai xg300 2001. While driving the will drop to 3rd gear and stay there, why

sounds like it isnt shifting all the way into 4rth, might need to check the shifter linkage or the stick there next to ur seat for tightness. but if its an automatic there isnt to much u can do

check the trans oil- in neutral with the engine running at operating temp. if its low or discoloured (should be red) change it, if not take it to a mechanic.

There r a few technical service bulletins from Hyundai on this concern, but only a dealer is in possession of them & the tools needed for the repair. Most times, it is a reprogram of the PCM or TCM computer, the need to reset & relearn the adaptive values. This makes sense if u understand that the transmission 'learns' how u drive & adjusts accordingly.

*AUTOMATIC TRANSAXLE HARSH AND/OR DELAYED UPSHIFT OR DOWNSHIFT - GDS ANALYSIS
*AUTOMATIC TRANSAXLE CONTROL MODULE - RESET AND RELEARN ADAPTIVE VALUE

Worst case scenario u r looking at replacing the entire transaxle. Many times the gears can be replaced if worn out but u have to pay a shop labor for taking it apart to find out. Often there is a trouble code in the history of the computer, even though ur check engine light is not on.

If u r the original owner, u have the 10yr/100,000 mile powertrain warranty & 90% of problems with the tranny would be covered.

If u r the second owner or more, l would suggest making an appointment with ur local dealer. If it does just need a reprogram it should not take more than an hour or cost more than $100.00 (depending on the shop is labor rate).

You have a shifting linkage problem. Yikes! It looks like my xg300. l took it to a professional and told me replacements of expensive parts has to be done. l sold the car and got a Civic.

l had a same problem on my Sonata .Problem is coming from speed sensor.When you drive over 40-50 Mph your transmission downshift from 4th to 3rd gear,check engine light comes on and gears wont change on their own.This is very common problem with Hyundai vehicles.Repair is not that expensive around $200.00 .In the future get ready for crank shaft sensor and cam shaft sensor.Before you get to spent lot of money always do some research .Info is out there just need to look for it.