Today while at highway cruising speed, I noticed that my check engine light came on. The car has no noticeable symptoms such as rough or irratic idling, no vibrations from misfire, and no hesitation or loss of power.

Previously I had the car looked at for a rough idle which was from misfire, and I went through a set of new wires, plugs, one coil, and gas and air filters. The last fix were the plugs, and the car seemed to run great, with none of the rough idle or vibrations.

Not sure what's wrong with it. I want to watch it a couple of days (and see if the light will fix itself) before going back to the mechanic. Any ideas??

I'd have it checked at Autozone, its free, and will tell you what is wrong, no use trying to pick at the problem, or letting the light work its way off. Could be an 02 sensor? Mine gave a CEL driving one afternoon because of the O2 in the manifold, worth looking up i'd say, and go from there.