Go to the Corner Auto Parts Store and get the code(s) read. Then post them up. Btw, a gas filter cleaner additive? Sounds like BS to me, and probably costs nearly what a filter would cost. A filter, good, bad, or otherwise is not going to throw any codes, as a matter of fact, nothing in the fuel delivery system will, except of course, a low fuel warning light!
And since you're new here -
Welcome to the Org!
A little something of significant value I put together for all the Newbies -
http://newbeetle.org/forums/new-member-forum/51021-newbie-welcome-kit.html
MORAV