Thats the $50,000 question. It is not easy diagnosing todays engine faults because sometime all you have is a code and it will not point directly to a certain device. Did you drive your vehicle at least 50 miles and a couple of days to see if the check engine light goes out now that you tightened the fuel cap? The computer will not automaticaly reset the check engine light if you tighten the cap. It has to go through a certain amount of miles and cycles so it knows the problem is fixed so don't be too fast to rush to another fix here. If it stays on I start with an O2 sensor since it has a lot to do with emissions. That is where I go and from there diagnose that to see if it is working properly. Best I can do for you now...............