Here's the story: about a month or so ago I first got the dreaded P0420 code. Thinking that because the car was 17 years old that it was probably time for a new cat assembly (2005 G35 coupe with 151,000 miles). Replaced the cat with a high flow cat from Walker and now the code remains..and now with P0430. My first thought was this was that maybe the cats were not bad and instead the oxygen sensors were bad, seeing as how they too were 17 years old. After doing a diagnostic I found these values for the voltage output of the sensors:

Bank 1, Sensor 1: .0.290v
Bank 1, Sensor 2: 0.340v
Bank 2, Sensor 1: 0.580v
Bank 2, Sensor 2: 0.340v

My understanding is that the values should be within 70% of the upstream to downstream values but I can't find anything which states what is a normal value at operating temperature while at idle, which is what these figures represent; heck I can't even find what the range of the voltage which lets you know if they are good in general. I suspect that this is my problem but without any other information I'm not sure. I am ordering all four new sensors but any thoughts are appreciated. It could be the new cats but I suspect something else, i, e the sensors as there are no perceptible exhaust leaks, rough idling or loss of power

 

Biggest culprit is an exhaust leak at the headers or cats.

Also check your spark plugs, if you're not fully igniting fuel it's going to throw the whole system off as it's constantly making large adjustments.

If those two items are good then get the engine up to temp, park or neutral, hold at 3000 rpm for 1 minute, then clear the code and see if it comes back. Try to narrow down exactly what conditions causes it to come back.

After that the next thing would be to pull the injectors off the plenum, put them back in the rail, then turn the ignition to ON to pump up the system and watch to see if any are dripping. A leaking injector like that can put raw fuel into the exhaust and throw everything off as well but you should be able to spot any differences in the spark plugs to see if one cylinder is misbehaving.