Help! My daily driver is turning into my Valiant!

The Valiant WAS the daily until it self destructed, I got the GMobile so I had something to drive while I worked on the Valiant. To be more precise, the car seems to start and run fine(though, the check engine light will come on if the AC is left on while the car is not moving. Not really a big deal, as fall and winter are almost here.). I actually took it on a 120 mile round trip with absolutely no problems. The light really doesn't seem to affect anything, but I want to be sure. I'll likely take the car by Advance auto and have them test the alternator.