I have an 02 maxima and just had the P0420 code pop up a couple of days ago. Within the past few days my car has gotten noticeably louder. Just idling its louder and hitting the accelerator makes it a lot louder than it used to be. I know its probably a cat problem, but my question is which one? By my research my car has 3 different catalytic converters, front left, front right, and rear. Any idea which one I should replace first?

 

Don't assume it's the cat. It could be any joint or piece, and most likely it is not the front portion since the rear is cooler and has more moisture. I had the hangar on the midpipe come off the pipe and that hole was about a nickel size but could sound very nasty.

Plus, cats are tons more expensive.....

Edit: just noticed you had a code. Sorry, then maybe it is the cat. Not familiar with that code.

 

That code is the firewall side cat (bank 1). It is most likely damaged and clogged or rusted giving false O2 readings. If it went bad inside, you must find out why it went bad. If you don't fix the issue that caused it, you risk damaging any replacement cat installed.

You can get front cats for about $125. I just installed Pacesetter cats on my Altima 3.5. You may need oxygen sensors, about $50 each. I was lucky enough to get the old 2 downstream sensors out.

 

Thanks for the reply, thats the one I was gonna try first