Hi y'all. I first want to say thank you to all the members who have put in work to help solve and diagnose common issues with our lovable Gs.

Onto the issue at hand. 2005 G35 Coupe is throwing a P0300, has a rough idle, and has a rattling noise coming from underneath the footwell area only at idle. Recently did a spark plug change, and ensured all the connectors were on the right coil packs. Car makes no rattling noise on acceleration and at speed. I do feel a loss of power in the higher revs compared to when this code was not present. All diagnostic procedures in the service manual (ignition, spark/coil, and fuel pressure) passed. The only test left was the cats.

I suspect that one of the cats is clogged (Bank 2).

Here is what Torque is showing for O2 voltages (as soon as I raise the rpms to ~2000 , the pink line shoots up, yet all other reading remain relatively unphased):



If someone can chime in if I am going down the wrong rabbit hole, or if I am overlooking something, I would greatly appreciate it.

Thanks.

 

A partially clogged cat can definitely cause a power loss. Not sure about also causing a P0300 code. Depending on your local laws and emissions testing, a set of downpipes could solve your issues if that's something you were considering anyway.

 

Downpipes really stink for a car you will drive in traffic. I would recommend Hfc's unless yours isn't a daily.

 

Clogged cats is probably not going to cause a multiple misfire. Purge the system of fuel, pull out all the spark plugs, hook them back into their coil packs one at a time, wear a leather glove and hold the assembly by the coil pack end while you keep the ground strap of the spark plug about 1/4" away from the intake manifold while someone else cranks the motor so you can VISUALLY INSPECT the spark for each plug. You probably just have a failing coil pack.

The data from your O2Bank2 sensor looks like the sensor went bad as well, no fluctuations just a flat line.

 

Also you can always take the cats off and visually inspect them as well, usually they look like melted droplets on the intake side if they're bad. Sometimes it's gone as far as a giant melted puddle.

 

Cats went bad on my first Maxima, you could see the substrate breaking up on the surface. Got a used pair from someone who put headers on their Maxima and my car was back to normal and running strong again.