Does anyone have a guess as to why my '01 civic keeps getting a check engine light & hesitating on aceleration

The dealer has read the check engine code multiple times and changed the gas cap multiple times. The code is always an emissions code. The issue has now extended to rough starts and cutting in and out while accelerating. The dealer can't seem to even guess at what it could be. Could this be an O2 sensor issue or a fuel/gas filter issue?|||Also, check your air filter. That could be an O2 problem that can be easily fixed by replacing it (or cleaning it, depending on what kind of filter you have) with just a few bucks.|||It sounds like a O2 sensor, or a MAF sensor. All you can do is just guess.