my alternator went out and i got it replaced yesterday. this morning the battery light and red brake/door lights on the dash are still on and not going away. i start heading to the shop this evening and the radio starts to cut out and lights start to fade on the dash as if alternator is bad again. then the transmission kicks out of gear and is in neutral as i pull over. after a few seconds the red lights go out on the dash and the car is back in gear but now the check engine light is on.

i took the car back to the shop and they say it's a bad alternator so they want to replace it again.

any idea why the check engine light went on?

 

no idea..your just gonna have to retrieve the dtc (diagnostic trouble code) and go from there depending on what code is set