I have a 2000 Altima (95,000 miles on it)that I recently bought. 300 miles later the check engine light came on. I had Auto Zone check it and the code showed knock sensor and catilic converter. Then I took it to a Nissan dealer and they checked it and said it needs a new converter. Nissan charged me 90 dollars to tell me the same thing Auto Zone did for free- Nissan wanted almost 900.00 to install a converter. I had a conveter installed elsewhere . Ran good for almost 400 miles and now it is back on. Before I got the converter installed the light would reset after a while and come back on. It would go back off after a while and then back on again. This went on for 5 cycles, each time the light comes on the gas tank level is around a quarter of a tank. Could there be a link between fuel level and this problem or do you guys think I got a bad converter installed. According to Auto Zone it is showing a bad converter and knock sensor again. It makes me think something else is causing the converter code. Any suggestions. I installed a new fuel filter today since I am going through it as I get funds to ensure it is in top shape.
Thanks
trhammer

Some fault codes can mean one or several items can be the culprit. knock sensor codes will come on when other codes are present. Whenever I have a EVAP code I also get the knock sensor code, it goes away when the EVAP code is repaired.
What specific codes are you getting? It should be at the top ot the Auto Zone receipt.