I own a 01 Chevy Impala. I had a mechanic shop put an after market converter on my car for me. It cost about $200. Later my check engine light came on. It was because of the cat. The oxygen sensor was saying it wasn't getting enough oxygen because the cat's chamber was too small or something like that. I have a 1 year warranty on the cat, so I asked the shop what they could do. They said it is a low end cat, and its been known to do that. They said they weren't sure if it would be covered by warranty because nothing was wrong with the cat - It didn't stop working or break. They said it wasn't affecting performance. They said they would look into it, and hinted that I could use the $200 to upgrade to a better cat. A GM cat costs $600. They never called back, and I was so frustrated I haven't called them back for a month. Should this be covered by warranty? Do I have to have them do the next install or can I get a refund to have someone else put it in? What should I do?