2004 hyundai accent code p0734?

P0734 is a code for incorrect 4th gear ratio. When the trans controller senses a disagreement between the input/output shaft speed ratio it wants for the gear it has selected and what it's seeing it sets this. If this condition is detected three times the trans locks to 3rd gear limp home mode assuming there is a serious internal problem and 3rd gear is intentionally designed to be the running gear with minimal control functionality so you can get home with most failures present. When it's in limp home it will accelerate like a dog because it's starting in a high gear and will wind out at speed without upshifting to 4th. Basically you're driving a car with one gear. There is an overdrive switch in the system but it's the switch on the shifter to deselect overdrive (4th gear) when going up grades so the trans won't hunt gears too much. If that switch fails it will only prevent 4th gear if it shorts or won't block 4th gear if it's open. There is no overdrive sensor. There is a kick down switch but it's part of the trans. If one of the shaft speed sensors was acting up you'd have a different code for it in most cases. Almost all causes of shaft speed mismatch are an internal mechanical problem, probably a burnt clutch, or an internal electrical problem with the solenoid valves. It is very unlikely there is some cheap external quick fix for what it sounds like yours is doing. Sorry but I'd be back at the clown you bought it from pronto. It sounds like you have an internal hydraulic or clutch problem that is aggravated by temperature as the fluid and components heat up. The chance of this happening without their prior knowledge is next to zero under the conditions you've stated. Sorry but pretty much any of the incorrect ratio codes are bad news.