I recently starting having check engine light coming on & off on my 2003 350Z. I also noticed some exhaust noise coming out from under the middle of the car. I took it to Advance Auto & Auto Zone to check the error codes. They said it was misfire on multiple cylinders. I asked which ones & no one could tell me. So I took it to my go to guy when I can't figure it out. He said to go to the Nissan dealer & they could give me that info. $145 later they said they didn't see any misfire codes but said my bank 1 02 sensor was giving erratic readings. When they pulled it out that said there was debri in the catalytic converter & said I should replace the converter & 02 sensor. So I checked prices online for both items & wow, what a difference in prices of both of them. I went on ahead & bought the 02 sensor & replaced it. The light stay off for several minutes but came back on & as I drove it seemed to not have the power when I put my foot into it. So I also replaced gasket between the y-pipe & rear exhaust pipes, but that didn't fix the leak. I checked with go to guy & he said that the the leak in the exhaust could be causing an issue with the 02 sensor reading, so I'm lost. Anybody know if this is possible? Also, my exhaust system is a Nismo system too. If I have to replace the catalytic converter, is there any sort of warranty left on my Z with 115,253 miles on it. If not then if I go online, amazon, ebay with some many different prices & styles, what should I do? I would prefer to have a bolt on placement, not particularly a Nissan replacement. I appreciate any advice from my fellow Z owners.