BMW: 1998 328i, gas cap, scan tool

Question
I have a 1998 328i and the check engine light keeps coming on.  When I have the code checked, it says the gas cap is loose.  I've reset it and it still keeps coming on, so my BMW mechanic sold me a new gas cap and this didn't help it.  He said it could also be the catalytic converter, but I hate to spend that much on it.  Actually, I want to fix the light once and for all so I can sell the car.  Please help......  I'm pulling out what hair I still have.  I heard that overfilling the gas tank can cause this, is there any truth to this?  Thanks!

Answer
Joe, this is a common problem with several cars.  There is a pressure sensor somewhere in the system that knows if the gas cap is not tightened.  Either this "sensor" is faulty, you have a hose that allows pressure to leak out, or your cap isn't being tightened 100%.  DO NOT let them tell you it is the Catalytic Converter, I would bet money that its just a ploy to make your wallet lighter.  
 I personally have not had this problem, but I highly doubt the catalytic converter is the reason you are getting a "gas cap loose" code.  Most guys at the dealerships dont know how to fix/find problems on their own, they rely soley on the scan tool to tell them what's wrong with the car.

 So Make 110% sure that you are tightening this cap all the way.  Dont break the cap, but make sure it's tight.  Have another dealership tell you exactly what the codes are that are coming up... look at the tool if necessary.

  Hope this helps,
 Josh