Hyundai Repair: Elantra no 4th gear, fluid temperature, transmission overhaul

Question
Hi, HT, My 98 Elantra,123k miles, if I release gas pedal at around 30 mph and push it again, the car refuse to accelarate with roaring of the engine, mostly happened in the morning. If turn OD off, it is fine, direct accelerate to 50 mph is fine too. I got code P0712, is it the reason for the problem?

Answer
P0712 indicates a problem with the readings from your trasnmission fluid temperature sensor.  Often this occurs because the wires at the connector for the fluid temperature sensor are broken.

Unfortunately, the issue with the temperature sensor is separate from your other issue, which is that your transmission is slipping in fourth gear.  Chances are high that you'll need a transmission overhaul or replacement.