I need a second opinion

I need help. I have a 2004 RX8. I took my car to the dealer because the cel was on. Also my car started to stall if I was driving on the freeway for about 30-45 mins then came to a quick stop on an exit. If I gradually slow down then stop, I don't have that problem. I suspected that I had a vacuum leak again. They did a diagnostic and told me there were no vacuum leaks and that I need a new engine. I'm not convinced. My engine was replaced in 2012 and it has about 53,000 miles on it. The codes that were read were P0171, P0442, and P0443. 0442 to my knowledge is linked to a vacuum leak, but they said I don't have a vacuum leak. I'm confused and I don't know where else to look for an answer.