The Nissan Path

06 Pathfinder LE. I have a problem I can’t seem to get rid of. This may be a little long but I want to give you the background. Three years ago we lived in Colorado at 8000 feet. Everything was good. My wife went to Las Vegas, 2500 ft. to visit relatives and the SES light came on. Got it checked at a local garage and it showed P0430, Catalyst efficiency below threshold bank two. Not a big deal so she drove it home. Before she got home though, the light went out. I said Yay. About a month later we decided to take out kids to Disney Land. Drove through Vegas to visit the relatives again and the SES came back on with the same code. We went on down to DL and the light went out again. Weird. Back through Vegas and SES is back on and then went home to 8000 feet and SES turned off. So because of some medical reasons we ended up moving to Reno. When we did, the SES lit again. I had to pass emissions so I researched it and found several things that could cause this problem. Intake Gasket, bad O2 sensor, Bad coil, bad Catalyst converter, etc. So I don’t like taking things apart twice and I had some extra money so I changed them all. I got new cats, new sparks, new intake gasket, new intake collector gasket, coils, injectors, fuel/air ratio, O2 sensors etc. I replaced every hose that looked even a little deteriorated. So after doing all that, the engine has never run so smoothly. But a week later, the SES comes back on again. This time it’s saying P0420, Catalyst efficiency below threshold bank one. So I checked all the bolts for the Cats. I had a bit of a problem getting the AF sensor to go in smoothly so I pulled it out and ran a chaser through the threads and put it back in. No Joy. I replaced the AF sensor and the O2 sensor on that side again wondering if one was bad. SES still lit with the same code. It still runs great. The MPG seems a little low. I’m at a loss. Any suggestions or ideas would be greatly appreciated.

I have both codes active on my 05. P0420 and P0430.

I have a Superchips Cortex 4950 tuner as well, which allows me to observe the voltages for the Front and Rear O2 sensors. I noticed that my rear O2 sensors seem to be quite erratic. But no issues with the way the truck runs. Exhaust smells normal, fuel economy normal, etc etc.

I have replaced the AF sensors before, about 150k kms ago. They read pretty consistently, and do not fluxuate much at all.

I think it I was to change my rear O2 sensors, the P0420/P0430 might go away. But I haven't tried it yet as its a $300 fix.