I'm new here and have a 2003 Nissan max SE with a P0420 code. I have done some research and most people say it's catalytic converter and O2 sensor but my question is which one exactly?

 

probably the pre-cat. almost every maxima owner gets this code at some point.

 

Is it this one?
http://www.partsgeek.com/gbproducts/...FY6e4Aod1lsA1Q

 

no, that's not it. there's another one underneath the car made into the downpipe directly below the engine.

 

yours should look like this...
http://www.courtesyparts.com/20020-t...1957_1981_1982&

 

I'm pretty sure on the 03, the rear pre-cat sits above that pipe and is not part of it. Google Eastern Catalytic part number 40477.

 

The 2003's have a 3 way cataylist system 2 pre cats and 1 main cat the code P0420 is pointing at the Bank 1 pre cat (which is the one closest to the fire wall). the pre cat you have in your link is for Bank 2 which is closest to the Rad.
http://www.rockauto.com/catalog/x,ca...,parttype,5808
this is the part & number from Rockauto .. EASTERN CATALYTIC Part # 40477 that you would be looking at replacing.

Mind you I would probably suggest changing the 02 sensor first as it might just be getting lazy and causing the P0420 code

 

you could be right, i was just going by courtesyparts diagram and i know P0420 is not the one by the radiator.

 

had 420 code for 2 years, kept erasing and coming back every 500miles or so. tried seafoam through the brake booster line, and erased the code never to appear again. That was over 60000 miles ago. Might be worth a try?

 

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You looking to get what "Ghost 54" mentioned if your going to get an Aftermarket pre-cat (half the price, but gaskets won't fit), & If your going OEM then your looking at part number 20802+A (Picture below - Firewall side) $567.08. Also would suggest checking your 02 Sensors first...

 

Hey guys I've got the P0420 code and I've been searching the forums for some good information. You seem to have a good discussion of which part needs to be replaced.

My first question has to do with the 20802+A pre-cat compared to the Eastern Catalytic 40477. The shape appears to be different. It doesn't have a bend to it. Is that what you mean that the gaskets won't fit? How do you handle the difference?

Second, my car seems fine at low RPMs (though not much power) but when I get to around 3000-3500 RPMs I get a chatter that sounds like what is shown in this video:
I don't see the issue while parked, in neutral revving up. It's more when I try to accelerate quickly or when I'm going up a large hill at highway speeds.

My car is at 212k miles. I've got a new air filter and just replaced spark plugs. I'm trying to determine what my next step is. Is it likely that both the pre-cat and main cat are bad?

 

^ What year is your car? That does not look like 5th gen, neither 5.5

 

Mine is a 2002. The video is just to demonstrate the sound I hear.

 

Installed a warpspeed Y pipe and a non fouler spacer on the O2 sensor, have not had the code since.