P0734=A/T 4th gear incorrect ratio. This has nothing to do with the speed sensor. If the speed sensor was faulty, it would also set OTHER gear incorrect ratio codes. My experience tells me the 4th gear incorrect ration indicates the torque converter clutch (TCC) is slipping and nothing more. Unless there was a speed sensor DTC (Diagnostic Trouble Code), the sensor is not the issue (yet). The fact P0734 was set (trust the computer), the TCC is slipping (out of range). Question: Maintenance history may indicate the wrong transmission fluid may have been introduced. Hyundai's Transmission fluid is strictly SPIII... there is no substitute. The incorrect fluid will cause 4th gear slippage. Now, having said that, it doesn't mean the 4th gear (lock-up) clutch isn't worn out. Have a qualified/certified Hyundai Service Technician properly diagnose this issue.
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