2002 2.4L P0420 code help please

Hey guys. Brand new to this site and I’ve been stumped with my truck for over a month now. I have a 2002 2.4L and I’ve had a check engine light ever since I put on a mbrp cat back exhaust. It keeps popping a P0420 code and I’ve fixed everything associated with it including 2 new cats and 2 new o2 sensors. When I first put on my exhaust I ran out of time since I’m here on camp pendleton and the skills center I was using was closing. So I ended up straight piping it pretty much with a pipe behind the first cat that had a port for the downstream o2 sensor. Made it back to the barracks without a light and then drove it again and the light popped which I figured it would. Ended up getting on the exhaust and everything right the day after and the light still stayed on. I thought driving it would fix it. I was thinking I might’ve fried the sensor but didn’t realize that till now. Took it toyota got a new cat the 1st stage to be exact. It stayed off for about a week and then came back on. Took it to a muffler shop to get a new downstream o2 sensor and 2nd stage cat welded because it seemed like the only option. Got that sorted and it stayed off for a day before coming back on. Muffler guy was super cool and ended up getting my truck scanned by a mechanic next door and it said slightly low voltage in the downstream sensor. Replaced it for free with another on from o Reillys and it came back on about immediately. Ended up replacing the upstream sensor and it came on again after the light was reset. Been stumped on this. Few things to keep in mind. Downstream sensor was an oem one from Toyota before being replaced by the muffler guy with a denso one. Upstream was a denso and was replaced by a denso. Front cat is oem from Toyota, 2nd cat was welded on with a cali carb legal from magnaflow. When the mechanic scanned my truck both cats were reading very good. So no problems there. Mechanic recommended flashing the ecu. What I’m thinking is my downstream o2 sensor needs to be oem from Toyota since it was originally like that. I feel my ecu is reading it with the voltage being off and popping the code. I also asked a few other mechanics over here and they said to take it to Toyota since tacomas are pretty specific about toyota parts especially sensors. Just wanted an input to what anyone thinks before I try anything. Also to note truck runs pretty much perfect besides a slight low idle sometimes but it’s always almost had that. I also think it might be the downstream sensor since I know that one has nothing to do with air/fuel mixture and can trigger the cel with a P0420 code. Thanks in advance to whoever replies!

 

Too much to read. Make sure there's no exhaust leak

 

There’s not I have it welded

 

So it's welded from the manifold to the exhaust tip? Even a minor leak from the oxygen sensor can trigger a p0420 and cause cat monitor to be not ready.

 

Its my understanding that Toyota OEM O2 sensors were (are) produced by Denso. So they are the same except for branded packaging. If you have replaced both sensors (DENSO/OEM), 1st cat (OEM) and second cat (magnaflow), cleared the codes and still getting a code... I would investigate the following:

• A malfunctioning catalytic converter is the usual suspect for a P0420 code (Magnaflow)
• Damage to or leakage from the exhaust manifold
• Damage or leaking in the exhaust pipe (or gaskets or O2 sensors)
• A malfunctioning oxygen sensor in the back or front
• Damage to the oxygen wiring
• The O2 wiring has a loose connection

I have the same year, color, engine and ended up replacing my entire exhaust system and 02 sensors after my header cracked, gaskets leaked and muffler rotted out. Its easy to do yourself. I replaced everything (including cats) with OEM and it was expensive as hell ($3K). Tacoma's are picky with air/fuel/exhaust/electronics parts. Many posts on here start with "I used an aftermarket rockauto sensor but im still getting a code".

https://www.way.com/blog/p0420-code-what-it-is-and-how-to-fix-it/#:~:text=What is a P0420 code,difficult to diagnose at times.

 

Remove your Idle Air Control Valve and clean it thoroughly with carb cleaner and a proper brush. If it's "almost always had that", the cleaning may rectify that situation and give you a solid idle.