Question about 2004 Mazda RX-8

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

Code po420

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Hello and welcome. the code P0420 is for a bad cat. converter. replacing 02 sensors wont repair this code. the code simply's means that thje cat. converter is not working as designed and will need to be replaced.

The PCM determines the oxygen storage capability of the catalytic converter has degraded below a calibrated threshold.

Posted on Jul 03, 2010

  • rx3140 Oct 29, 2010

    What is or are the long term possible damage caused by driving with a defective downstream(the last 02 sensor)?

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I have 2 catalytic converters on my 1996 Buick Roadmaster, I know 1 of them is bad. How can I tell which 1 it is so that I can buy the right one?


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

Jul 26, 2013 | 1996 Buick Roadmaster

1 Answer

2003 toyota matrix with code po420


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

Jul 09, 2013 | Cars & Trucks

1 Answer

Check engine light comes on, will be on for couple days up to two weeks then goes off only to come on again after a while. code po420. I've replaced both o2 sensors and replaced the gas cap, the...


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.

Jul 27, 2011 | 2000 Toyota Camry

3 Answers

Readings on my 2003 Toyota 4 cylinder ck engine soon is P0128, P0420, P0430.. what could it be. Should I change thermostat, 02 sensor or catalytic converter.. Please advise.


PO128 = It is probably a bad ECT (engine coolant temp) sensor. PO420= The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail.PO430= Catalyst System Efficiency Below Threshold (Bank 2) Your CAT converter is not cleaning properly. Hope this helps, James Booth

May 17, 2011 | Toyota Highlander Cars & Trucks

1 Answer

Code P0420 is it an oxygen sensor or the catalytic converter?


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.

Mar 19, 2011 | 2003 Honda Element

2 Answers

Catalyst system efficiency below threshold


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

Jan 17, 2011 | Chevrolet Astro Cars & Trucks

1 Answer

Po420 bank 1 and po430 bank 2 error codes


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.

May 13, 2010 | 2007 Ford Mustang GT California Special...

2 Answers

Code PO420 catalyst system efficiency below threshold


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.

Mar 05, 2010 | 2001 Nissan Altima

4 Answers

I have a 2001 Santa Fe and the Engine light is lit and has been for many years. When I took the vehicle for the State Inspection I was told that the Co2 needed to be changed so the light would come off and...


The best thing to do is go to Auto Zone and get your Hyundai scanned for a DTC code. That would tell us which O2 sensor has gone bad. You have three O2 sensors and one of them has gone bad, the O2 sensor sends signals to the ecm to control the air fuel mixture so it will not run to rich or to lean which is causing you to fail the state inspection. Good luck and keep me posted on what the DTC codes are, be glad to help.

Jun 25, 2009 | 2001 Hyundai Santa Fe

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