CEL code P0420 displayed despite replacing Catalytic converter and the O2 sensors

About a 200 miles into my used Toyota Tacoma (thing had 210,010 miles on it) the CEL light comes on. Its p0420, my catalytic converter not at optimal efficiency. OK whatever, Ill just replace it with a cheap one(first vehicle, no job, no money, etc.) I bought this

$46 AUTOSAVER88 ATCC0017 2.25" Inlet/Outlet Universal Catalytic Converter with Heat Shield (EPA Compliant)

catalytic converter and welded it in place of the old one. Reset the code, then it comes back about 20 miles later. OK, then it must be the O2 sensor. I bought the

$59 Denso 234-4206 Oxygen Sensor

denso 234-4206 Oxygen Sensor and put it in place of the old one. CEL gone!! for about 500 miles or so. So I'm back to square one.

>what do

This is my theory, I bought a non-CARB compliant catalytic converter because I don't live in places such as NY or Cali.
But when the catalytic converter was stock it was CARB compliant.
So the ECU is spitting codes because it is programmed to comply with the emission laws in Cali.
Can I reprogram the ECU to comply with Louisiana emission laws?
or would it be easier to buy a CARB compliant cat and reinstall?

 

Coolant levels normal?

 

@elduder Yessir. No leaks that I have noticed. Is a Leakdown test in order?

 

Not necessarily , just wanted to make sure you werent noticing low coolant in the reservoir. Coolant leaking by can trash a cat and foul a sensor. Not completely familiar with the 1st gen but I believe there is another 02 sensor upstream from the one you replaced.

 

@elduder nope that's a fuel sensor. i actually bought two 02 sensors and replaced them both(the upsteam and downstream) but it shot another code, something about heater wires, due to that i put the old upstream sensor back and that worked for 500 miles.

I looked at my entire exhaust system and didn't see any other sensors other than the two. Which leads me to think that the upsteam is a fuel sensor and the downsteam is an 02.