general problems of the 2004 Honda Civic

I have a 2004 Honda Civic sedan I took my car to scott robinson Honda in torrance California for a smog check without any mechanical problems my car passed the smog check without any problems. I left the facility one day later my car had the check engine light on. Went back to the dealer and they came up with a code that they said it needed a new catalytic converter . I became skeptical about this recommendation so I started looking through the owner's manual and read that an engine light on could be possibly a loose gas cap. So I decided to buy a new original fuel cap I went to the dealers parts department and purchased a new fuel pressure cap I noticed that the replacement cap did not look like the original but the part department person said that it would work just fine I then replaced the original fuel pressure cap. After a few days the check engine light turned off until a few days later the light came back on then I check the fuel pressure cap and it is loose I tighten it and in another day the check engine light is off. And it keeps doing this until now sometimes on sometimes off I also see that my fuel level gauge erratic it has been like this since I bought my car but I just did not think that this could be a problem I hope that something gets done about this thank you for your help.

Needed to replace fuel door cable. This seems to be a common occurrence.