Hey guys. I need some help with some issues going on with my g35. It’s a 2005 sedan base, about 216k.
For about 2 years now I’ve had a p0420 code and I’ve just said **** it, not sure if it’s o2 sensors or the whole cat is bad, up until about 4 weeks ago my car has ran perfectly fine.
so I was driving and came to a stop light, as I was sitting there the car started to idle alittle rough, I was thinking uhhhh okay maybe a misfire?? Well I started moving and my exhaust note completely changed (I have stock exhaust with muffler delete)
it honestly sounds like a 5cylinder, that’s the best way I can explain it. So I noticed the rough idle and exhaust note changed and lack of acceleration, but once up to speed the car drove perfectly fine, didn’t spit and sputter like it usually would with a misfire, it drives as if nothing is wrong but sounds like a 5cylinder and idles rough. So I was like well that’s weird because it didn’t though a code, only current code I have is the cat code.
So My first options were to replace all coils and plugs, but with the history of misfires I’ve had with the car, it drives like complete crap, and it throws a code. Well none of that is happening, so I’ve been told it’s most likely not a misfire, some were telling me to do a compression check, haven’t gotten around to trying that and if I do I I might as well swap all new coils and plugs while I’m at it. Some told me head gasket, id like the pray it’s not. Now all those scenarios don’t really explain the exhaust note sounding like a 5 cylinder. Could it be? Or maybe a cat finally gave out on me?? I mean that would be the only explanation in my book on why it sounds like it does.
so I don’t even know where to begin any suggestions??

 

Probably smoked the cat, take it off and look down it (angle side needs an inspection mirror to look around the corner, or a borescope) and see if it's melted.

That code is not an O2 sensor failure code, it's a warning that you need to fix whatever is causing the catalytic converter's to not work otherwise they're going to be destroyed and cost quite a bit of money to fix.

 

Also, pull and inspect your plugs, you might be down a cylinder and you'll see it in the plugs.