Passed my smog test, and soon after code P0420 displayed?

2010 HONDA ACCORD
145,000 MILES • 3.5L • V6 • 2WD • AUTOMATIC
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SIMON.SAYS
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Passed my smog test, and soon after the P0420 displayed, took it to dealership and they said there is no problem with CAT converter and did a $250 inspection and provided me a report, and reset code, drove another 100 miles ,P0420 displayed again, I went and filled up a better octane gas and drove the Code P0420 went out and now running without any problem.

O2 Sensor(Upstream) has never been replaced since purchase, read that due to wear and tear and wires and 02 sensor can degrade, I am planning to trouble shoot this intermittent issue with first replacing the O2 Sensor (Denso 234-5098 Air Fuel Sensor ) which is the same as n 36531-R70-A02 Sensor, Laf Front 2008-2013 for Honda Accord 2010 EXL v6

Appreciate if the your experts could provide any guidance and tips. TIA
Oct 12, 2024 at 2:41 PM
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STRAILER
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The P0420 code on a 2010 Honda Accord refers to a "Catalyst System Efficiency Below Threshold (Bank 1)." so this means the cat is getting weak. You can try to replace the secondary oxygen sensor on bank 1 to see if that helps, but if not, the catalytic converter will need to be replaced. Here is the location for the sensor and how to replace it. Check out the images (below). Let us know how it goes.
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Oct 13, 2024 at 10:01 AM
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SIMON.SAYS
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Ken,

You are very kind thank you very much for the guidance and advice, as per your suggestion just changing the 02 sensor you have marked in "red" will take care of this, ass per the first diagrams , there are 4 02 Sensor in 2010 Honda Accord EXL V6 (USA), I will start with your recommendation of changing the identified, and then look at the others. This Accord is my second one , my first one was 1992 Accord , i drove it for 250,000 miles and donated it like new these cars are made real good, my decision to go for the V6 EXL was to just boost my enjoyment of riding, please understand my current car is running like new still , it is this annoying codes which are popping up, and Honda know about it . I want to thank you and 2CarPros, to build this website where a honest diagnostic troubleshooting with Pros helps owners of Honda Accord user, I stumbled upon this website after reading other website, where I did not find "Actionable" mitigations were clearly described like you have done. I will update this post after following your advice.

Can you provide me the exact part number for this O2 Sensor, is this sensor same as O2 Sensor (Denso 234-5098 Air Fuel Sensor) which is the same as n 36531-R70-A02 Sensor, Laf Front 2008-2013 for Honda Accord 2010 EXL v6, I mentioned?

Thank you.

Additional Info: I checked the Service Manual for the DTC, also check all the recall for.
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Oct 13, 2024 at 12:26 PM
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STRAILER
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Thank you, we like the site as well. I believe the part number for the downstream O2 sensor on this model is Denso 234-4349.
Oct 14, 2024 at 9:49 AM
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SIMON.SAYS
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Thanks.
Oct 14, 2024 at 9:51 AM
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STRAILER
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You are welcome, please use 2CarPros anytime we are here to help.
Oct 14, 2024 at 10:23 AM