I've searched all over and despite others asking the same question about the notorious p0420 code and not sharing info on their cars, I thought id try for specifics.

2006 nissan 350z vq35de 3.5l v6 6sp
42,000 miles - synthetic oil:

Issue:

1. P0420 code "catalytic efficiency below threshold bank 1"

2. Eats premium..roughly 13 mpg or 180 miles per tank in town. (temp daily 90+ with florida humidity)

PERFORMANCE UPGRADES:

Jim wolf popcharger, obx headers, megan racing hf resonated cats with oe o2 sensors, megan racing y-pipe, pp topspeed pro n1 dual 3" cat back exhaust and jdm oil catch can.

--what with that info could cause p0420 (i replaced passenger side downstream o2 sensor) and bad gas mileage/run rich? The exhaust is all new...stock ecu...please, any technicians opinion or fact welcomed...thanks alot again!!!

 

I'm no expert, could be wrong. ECU might be overcompensating for all those breathing mods? Normally i thought adding all that would lean a car out. Did you try resetting the ECU? Also, I would get that car tuned/re-flashed.

 

Well, I have reset the ecu by doing the peddle trick...no resolve...can a stock ecu be reflashed and by who?

 

I was looking at an Apexi super AFC NEO (350.00) to fix my running rich due to flow issue.. Anyone ever use these? Seems like it has good reviews.

 

Check for exhaust leak on the Passenger side. The leak most likely will be your Cracked HFC or the point where your header meets the the top of your HFC.

Also here is a search of related to your issue.

https://my350z.com/forum/search.php?searchid=14399440

 

0420 is NOT a rich/lean code, it's fairly normal for HFC and test pipes. The actual rich condition could be cause by an exhaust leak, cracked HFC, or even antifoulers/extenders/other tricks on your lower o2 sensors.

 

you need 02 spacers/antifoulers

you may/may not need an ecu tune as well, only way to tell is connect a wideband 02 and moniter various ecu parameters simultaneously to check timing curve, IPW, knock, etc. The need for a tune will not cause a P0420, which is simply a cat inefficiency code (ie, the ecu moniters how fast it takes the 02's to heat up, and what temperatures they reach. Because you no longer have cats, they heat up much faster than they do with factory cats installed, and that is causing your CEL). This is compounded by the fact that the car is a post 2004.5 model year, which are less forgiving to test pipes/high flow cats vs earlier year Z's

 

Try spacers first, I used to have a Tiburon with DC headers and threw this code and the P0430 code and luckily Spacers worked for me.
They are a cheap solution that may or may not work, just depends on the car

 

I ran the car and crawled under it. I did not note any leaks unless so fine that ears can't hear....but would extreme fine leaks cause as well?

 

I will try these this weekend. thank you!