07 G35s P0420 Engine Code Help!!

[tehgreatmg]

My engine light came on and I went to autozone to get the code pulled. It came up as P0420 Catalyst system efficiency. I did some research and most people are saying that is usually due to the rear O2 sensor getting different readings from aftermarket cats. My cats are still stock though and I have 68K on the car with stillen intake and cat back exhaust. I looked for vacuum leaks and couldn't detect any. I'm going to try swapping the O2 sensors and see if the code comes back on. My question is does anyone think it is possible that poor/watered gas could be causing this. I had to fill up at a small country station the day the light came on so that's what makes me think it could be funky gas. Also, does anyone think I should take it into the dealer and that this might be covered since I'm still under the 70k or will they tell me its the aftermarket parts fault? Any advice would be appreciated.

[SteveTheTech]

You need to stop before doing anything yourself. Your car is still covered under the Federal Emissions warranty, 8/80.

If I were you I would take it in to the dealer soon.
They might tell you that you will have to approve a diagnostic charge....it should still be warranty.