I had to take my car to a mechanic to replace right engine motor mount, and about a couple of days later whenever I start up the car, the gear changes from 1st to 3rd then stays there. The check battery light is on too. Does anyone know what is causing this?

 

Just worth checking. He may have left the engine block electrical ground off, or connected it improperly. If the PCM sees things with the engine are seriously out of whack, it will default to limp mode as a safe get you home configuration, which is why your gears "misbehave". Go back to your guy (who by the way may have deep-sixed your alternator voltage regulator too, if it is a main ground connection issue).

 

do you have an auto transmission? if you replaced a motor mount, ANY motor mount for that matter, you dont have to touch any sensors or the alternator.

do u have a 6s or 6i?