The Nissan Path

Hi new to forum,thought I would share this with members having this problem. I own a 2006 pathfinder ,about a month ago it started running really ruff, no power engine sounded horrible like it was dieseling ,I could feel a vibration in the engine kept giving a p0300, I changed front cats. Spark plugs and nothing so I took it to a mechanic ,after running multiple tests nothing, then he said lets disconnect the rear cats from the front and behold car revd like it should ,so far over 300 miles and no issues so far hope this can shed some light for some of you