Quest to repair P0420 code

[Dan.NY]

For many years now, my 1998 3.3 4wd pathfinder has been throwing p0420. I’m now replacing a leaking water pump and timing belt which brought me to this forum so once again I’m going to try and figure why this is happening. Hopefully with some help from this forum. Some history. It’s a bit of a tale so hang in there.  I bought this vehicle new in 98 in California. Yes it’s a California car. In about 2011, the distributor failed and it was a real pain to get repaired. I was not in a position to troubleshoot and repair myself at the time. So into the repair shop it went.  The clowns there couldn’t figure out what was wrong. This lead to many parts being changed, swapped, replaced. Ultimately I ended up with new cats, which I didn’t need, a new distributor which I did need and some other items (I think o2s). About three months later I threw the cat codes, reset and didn’t think much. This process Continued but the time between reset grew shorter. I looked into repairing this, even took to a few repair shops. Each time I was told it’s not possible to fix due to age, worn parts, etc etc. My cats were new when this began, so I never suspected them. Also o2s. I replaced the cv boots and after this, the code stayed off nearly a year. It came back with a vengeance and stays on nearly constantly now.  I thought there is a sensor near the rotors that may have been involved but never Figured it out.  I would like to begin getting rid of this code but am not sure where to begin. Any thoughts how to proceed, where to look?.thanks..

[Mattmill91]

I have a 97 - with 235k with no codes but a knock sensor.

I have changed the cats - o2s and charcoal can and valve.

[Mattmill91]

Do your self a favor and get a code reader - somthing that will allow you to view all the data streams coming off the 02s.

But some one correct me if I am wrong.

P420 appears to be a cat code-

The computer would throw this code if the data from the 02s pre/post on either bank doesn’t match what the computer thinks it should be.

The first 02 should fluctuate rather rapidly from rich to lean. But the down stream o2 behind the cat should remain rather constant.

[Mattmill91]

Here’s a screen shot of a data screen I had assembled on my iPhone- app is “engine link” and the device is “elm327” WiFi dongle

 

 

https://www.ebay.com/itm/ELM327-OBD2-Bluetooth-WIFI-Car-Diagnostic-Tool-ELM-327-OBD-II-Scanner-/153187642003?_trksid=p2349624.m46890.l49286

[Storm_Trooper]

I run one on each downstream sensor with gutted cats and don't get cat codes anymore when I had cats. 

https://www.amazon.com/dp/B08RHDLRHT?ref=ppx_pop_mob_ap_share