1996 Mercury Marquis O/D off false error code?

Hi, I need some guidance! A week ago I was driving my car and the passenger was playing with the o/d button. He kept pushing it on and off repeatedly. Now after I drive my car for 10-15 min and/or go above 45mph, the "o/d off" light flashes on my dash. I also have a "check engine soon" light flashing. I did bring the car in to have the codes read, but at this moment I don't have the actual code numbers they gave me. One person told me my 4th gear is gone and I need to have my transmission rebuilt. Is it at all possible I am getting a false code because of my friend messing with the button while I was driving? How could I test this theory. Is there a way to clear all the codes and see if they return? Any help or direction is much appreciated! Thanks!