I have one confirmed P0421 code and one pending code P0301

2004 HYUNDAI SONATA
90,200 MILES • 2.4L • 4 CYL • 2WD • MANUAL
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VISH4369
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This morning P0301 code pop up in the pending code category. I have P0421 code which is related to catalyst 1 efficiency below threshold Bank 1. I am not sure it it because of O2 sensor bad. How many 02 sensors or catalystic converters in this car? the reason I am asking this because there are two sensors after exhaust manifold before and after what looks like a catalytic converter. And there is another one which looks like a catalytic converter under the car after flex pipe exhaust. Is the catalytic converter causing the misfire? I have a crack after flex under the car is that causing P0421?
Jan 8, 2018 at 1:27 PM
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STRAILER
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Hello,

Yes, this code is typical of a weak catalytic converter. This test will confirm the problem so you can install a new catalytic converter. The P0301 code I would install new spark plugs.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

and

https://www.2carpros.com/articles/catalytic-converter-replacement

and

https://www.2carpros.com/articles/how-to-change-spark-plugs

Check out the diagrams (below).

Please let us know what you find. We are interested to see what it is.

Cheers, Ken
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Jan 9, 2018 at 4:47 PM
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VISH4369
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I was scanning for both O2 sensor volts and to my surprise both are reading almost same. The post O2 sensor which is supposed to be steady was reading almost close to pre O2 sensor and fluctuating a lot. I have a crack just on exhaust pipe just before central muffler after flex pipe. Is this causing them to read like that. Changing the whole catalystic convertor is too expensive for me. Do you think swapping the both sensors works ??
Jan 9, 2018 at 8:33 PM
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STRAILER
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Not really I don't think you can plug them in #1. A hole in the exhaust system will cause a problems with the sensors though.
Jan 10, 2018 at 12:28 PM