Need Help with 2002 sonata ..Tranny

hi guys im new but i did some research and it seams Sonatas From 1999-2002 have the same problem with the gears getting stuck in 2nd or third gear ...mine has just started doing it and i only got 36,000 miles its sluggish on takeoff and you can feel it shift it bumps or grinds ...Alot of people seem to think its a Sensor that causes this problem .does anyone know where these Sensors are located are they easy to switch out ?,and also is there a procedure to get codes from  the Car  because my engine light comes off and on .......
When i brought it to the dealer they said i needed to do a tranny flush ...so i did it and same old same ...any help would be greatly appreciated thanks alot..

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