My 2001 Kia Optima starts out fine then it will skip second gear and shift very hard into third and stay there. The dealership checked the car and said to drain and fill the trans, did it, didn't work. The trouble codes are " P0715" and " P0720" can I just replace the Input/turbine speed sensor and the Output speed sensor?
No, these codes are there not because the sensors are defective but rather that they have detected a problem. Thats what sensors are there for and seeing as you have more than one sensor pointing at a similar problem I'm almost positve you can rule out just the sensors. Sorry!