It could seriously be ANYTHING. I have been doing this exact job all summer long, and I have seen it for TONS of things. Hell, the new Lincolns will set a CEL if you start your engine with the gas tank not properly shut all the way. By what you are saying, it prolly isnt anything major.
Most common CEL's were:
IAC motors
PVC valves
DPFE sensors
mostly emmision type stuff.