I have a 1998 A6 quattro with 180,000 miles. Two things happen with this car. When I fill up with gas it is hard to get started...after initial start it starts up fine. Second problem, after driving around town all day, it is hard to start, like engine isn't getting enough gas....it sputters then dies. Once it is going it drives fine. Check engine light is on and mechanic says it is catalytic converter...is that causing all of these problems?

 

The converter could definetly be causing your issues after extended periods of driving but your problem with the hard start after refueling I beleive is another isssue in and of itself. do some searching here or over at audiworld I know I have seen posts about this issue just not sure where I read it.

If I remember right the cause of the hard start after refuel is linked to the evaporative system something about a check valve that sticks and lets fuel push back into the engine during refuel I beleive the way they recommend to check for this is to remove the charcoal cannister which is located under a plastic cover directly underneath the spare tire well under the rear of the vehicle. the cannister should be pretty lite only being filled with charcoal but when there is a problem it lets fuel push into it making it much heavier. They say the cause for this issue is trying to top off the tank while fueling it is recomended to stop fueling as soon as the pump clicks off. Hope this helps and like I said do a little searching you will find better explanations of this issue.