My daughter has a 2002 Elantra. WHen she is driving on the highway, the car will down shift by itself to 3rd. The check engine light will come on. If she pulls over and shuts off the engine the light goes off and the car runs fine. Any simple fixes that we may be able to try?

You'd better bring your car to a dealersip . the problem should be under warranty. Some electric problem in the tranny cause check engine light on and down shift. Good luck.

It could be a few things. First thing you need to do it get a scanner on the car to see what codes are being flagged. Other possibilities, other than internal transmission problems, would be a throttle position sensor, or a transmission range switch. If you can get a code for us, that would help out a lot.