I started my '03 Malibu the other day and noticed the engine light on. I drove it around for a while on the street and freeway and didn't notice any performance difference. I've driven it around now for a couple of days, but still haven't noticed any problems. I thought engine lights always came on when a serious mechanical problem occurred. What would cause an engine light to come on while the car still runs fine? Any help is greatly appreciated.

check the gas cap if its not tight. it will cause the light to come on (see owners manual)

or it could be the o2 sensor or something which is only for emmission control

Most likely it is the gas cap. Go to an auto parts house (like AutoZone) and they will read the code for free.