Po735 code on my 2002 Kia sedona what is this problem?

This is often found with a bad vehicle speed output sensor. As it gets worse the 5 speed auto Trans will downshift to 3rd while at highway speeds (very violently) and subsequent use to shift past 3rd gear unless you shut the van off and restart. Sensor us readily access as it is on top of the transmission and held in by 1 10mm bokt. You will need an extension with a swivel joint to reach it. Takes 15 minutes to change. Had this exact issue on my 01 Sedona back in 2006. Sensor was $100 at the time and a dealer only part.

P0735 KIA - Fifth Gear Incorrect Ratio https://www.autocodes.com/p0735_kia.html