Rough idle

What is your question?

My car has a slightly rough idle from start up through driving and the check engine light comes on and stays on. I have had a mechanic look at it and he seems to think this is being caused by a bad head gasket but I have no leaking coolant,the temp stays in the middle throughout driving, and i dont have any smoke coming from the car. So i was wondering if anyone knows if this could be a bad head gasket or something completely different? please please please give details of your suggestions, Thank you.