Code po420 on scan tool but o2 sensor was replace 2 months ago

You might have your catalytic converter flow tested.

If one is bad you replace both
You have a 1996 bad or not their
over 14 years old so they get replaced
at that point,as they no longer are efficient
enough to process oxygen
You use the PO420 & PO430 codes you should
have as well as look at scan tool data,fuel trim,& back
probe & test the O2 Sensors.
If you hit them by hand when cold they may rattle
as the material inside is broken prosibly
Average person can not tell much,so no
Need experience & proper test equipment
Just replace both side & the front O2 sensors yourself

You don't check O2 Sensors when cold,
they won't be operating or switching until
the computer goes into closed loop & converter
lights off at 600 degrees & continues to hopefully
around 900 to 1,100 degrees
Other than replacing a sensor, the only real
test I found of any value, is to watch switching
times on an oscilloscope
You can, as you said, look at scan tool data
Giving your vehicle is a 2003 & converters last
12 to 14 years, you know yours is worn out,doesn't
matter what the inspection gas results were & with
any codes, you should have been rejected & not tested
If you check fuel trim,you will see how well the MAF Sensor
is doing & you can't adjust fuel if the converter is not capable
of holding oxygen for the sensors to read
The front O2's get replaced on all vehicles at 100,000
when or before they slow down & fail

Bank 1 is side no 1 cylinder is located on bank to is opposite Sensor 1 before cat sensor 2 after

Here's a link to your P0420 code and what could cause it. https://www.autocodes.com/p0420_toyota.html

The after market cat is the likely problem. Catalytic converters are not all the same and over time our shop has learned which aftermarket ones will work and which ones will not.

The computer is looking at the downstream O2 sensor (the one after the cat) to determine cat efficiency. If the downstream O2 sensor is switching simular to the upstream O2 sensor (the one before the cat) than the computer interprets this as a bad cat. The rear O2 sensor should hold a steady voltage at idle once the car is warmed up.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

The code po420 is the cat efficencey which means the cat is bad but it could be a bad sensor, you will need to have the o2 sensors checked to see but lots of techs say a good tech should know if an o2 is bad but i will tell you i have had some that test good and end up being bad. they tested good at the time i checked them, a good rule that i follow any car with over 80,000 miles on it with a bad converter gets new o2 sensors, this is to help customer and us from comebacks, a weak or slow o2 will destroy the new converter, it may test good at the time of the new converter but then a few months down the road it gets lazy and after a while some ones not going to be happy having to buy another converter after just doing one a few months ago or even a year ago, seen it ,done it, many times,if i were you i would replace the o2 sensors and reset the light and go from there , if light stays out good if not replace the converter, all you are doing is doing it in two steps and you may get lucky and not have to replace the converter good luck.

is that the code you get from comp is PO420 eng light if so worst news either you have an exh leak between o2 sensor 1 and o2 sensor 2 or the front o2 sensor is lazy or the front catyletic converter is not working

basicaly code is only triggerd when engine is hot and the front o2 sensor and the rear o2 sensor read the same voltage when conv is working the front o2 sensor will fluctuate rapidly from 0v to 1v and rear o2 sensor will remain steady at 0.5v

try to explain best i can plz vote comment or reply if you need more help

sounds like O2 sensors to me. I havent changed one on a mustang yet but I havent had any trouble with any others that I have done, they screw in like a spark plug but you will need a special tool, I hope this helps you.

you may have a bad converter, its not cleaning the exhaust, but on the other hand a bad rear o2 sensor can also cause this problem,there is a wat to test but you would need a scan tool or scope,replace the other sensor, if light returns then replace the converter, no money lost, if you were to replace the converter the o2 sensors should also be replaced. you have 165000 miles it would be crazy not to replace sensor and install new cat, so basically your just doing it in 2 steps and may get lucky and not need a cat. good luck.