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.

 

I got the same code when i changed my cats to HFCs. However i got both codes for each side. 0420 and 0430. If your only getting one side, then it could be the o2 sensor gone bad. If the sensor is under warranty then it should be covered by the dealer. Since the sensor is for the cats and NOT the exhaust, dealer should not be able to blame on the exhaust. good luck.

 

my 07 G35s also threw this code around 50k miles. I took it in to the dealership, had a reflash, and it never came back. they claimed it was a "ghost code"...

now if I could get the p0011 to stop coming on...