mpd2434

Hi all i'm new to this sight and have my 1st question. I recently purchased a used 2002 Altima 2.5l. It has 68000 miles on it. The check engine light was coming on intermittedly for a week then stayed on continuously. When stopping abruptly the engine will sputter for approx. 10 seconds before smoothing out again. The code that my mechanic pulled was showing a problem with #2 cylinder. He stated the piston had alot of carbon build up on it. He is a local shop and can't seem to figure out what the problem is. Has anyone had a similar problem? I bought the car from a Ford Dealership and my warranty expired a week ago. They are refusing to help. Oh my mechanic said that someone previously replaced the spark plugs and some other parts so does that mean someone else may have had this problem? Sorry for the long post.