P0420 - Catalytic Converter code - Replace Cat?

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Hey everyone, I just bought a slightly high mileage Accord and when I purchased it, it had a check engine light on (P1157 - O2 sensor). I replaced it with an OEM sensor (Denso, I believe) and that code went away for about a day, then the CEL came back on the next day with a P0420 code, which from what I gather means the Catalytic Converter is not working properly or there's an exhaust leak. Is there any necessary reason to really replace the cat on an older high mileage vehicle aside from the fact that it may be giving off more pollution? In MI we do not have yearly inspections, but if there's a chance it'll damage the vehicle I may spring for one. Any thoughts? It seems to run perfectly fine. -knocks on wood- I've never had a Catalytic Converter go out on any of my vehicles, so I'm not very familiar with what I should do in this situation.
 
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As long as you can get an inspection sticker leave it alone Honda has had this issue on a number of vehicles mostly the ones that eat oil
 

JTK

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You could try swapping out the downstream O2 sensor with a new one. The system looks for the proper delta between the two, signifying proper CC operation (wants to see less O2 content @ sensor 2). Going with a new upstream sensor could have possibly changed this delta, setting the code. Spark plug anti foulers threaded into the downstream O2 sensor port can also help keep the P0420 away.
 
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ThirdeYe

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Originally Posted By: crazyoildude
As long as you can get an inspection sticker leave it alone Honda has had this issue on a number of vehicles mostly the ones that eat oil
I suspected it was running rich from having the upstream O2 sensor out for a while, thus killing the Cat... but I could be wrong. I am not really sure if it burns any oil yet as I haven't even had it a week yet. I've put about 300 miles on it and haven't had much of a chance to check the levels, since I don't have room to open the hood in our tiny garage.
 
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Assuming you replaced the cat monitor sensor, you can always stack 2 spark plug non-foulers with the top one drilled to push the cat monitor out just enough to turn the light off. Not in the spirit of the game, but it usually works.
 
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Yep, could be a bad cat or just a bad downstream O2 sensor. If it drinks oil then maybe the cat is bad.
 
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This a fairly common Honda code, the first thing you should do is see if there is a TSB and corresponding software update. Then start trouble shooting.... if the code is real, it could be a bad secondary sensor or a bad cat...
 
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Originally Posted By: ThirdeYe
I just bought a slightly high mileage Accord
572K is definitely more than "slightly." Got pics of this beast!?
 
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At your mileage you may just need a cat. But there is updated PCM software available to address a "false" P0420 on 06 and 07 Accords.
 

ThirdeYe

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Originally Posted By: mclasser
Originally Posted By: ThirdeYe
I just bought a slightly high mileage Accord
572K is definitely more than "slightly." Got pics of this beast!?
Yes, I need to get the plastic cleaned on the gauges. It was kinda dirty when I got it.
 
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Sometimes the downstream O2 sensor can lose accuracy and when that happens, it may trigger a P0420 even if the cat is fine. I'd replace the downstream sensor first because one accurate upstream sensor and one marginal downstream sensor can lead to a false P0420 code. However, given the mileage of your car, I wouldn't be surprised of both the cat and downstream O2 sensor needed replacement.
 
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Probably a traveling sales person. If it saw annual drain and fills from the dealer then it was probably treated pretty well. Plus it looks to be in excellent shape.
 
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I always thought that once you hit 300,000 miles you're driving on borrowed time, but I've been proven wrong numerous times and you are also proving me wrong. I'm currently at 195,000 miles on my CRV and look forward to 200,000 miles. I hope to be at your mileage one day and hope my vehicle will be as clean as yours when I reach your mileage.
 

ThirdeYe

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Believe it or not, everything works perfectly fine. Ice cold dual zone ac, heater works great, power leather seats, power heated mirrors, 6 cd changer works fine, engine and transmission are fine and the engine is super quiet and smooth. Previous owner changed the oil at the dealer every 8-10k and changed the ATF once a year at the dealer. No sludge under the filler cap. The valve train is pretty clean as well. The main issues I am having ith it right now are suspension related, along with these emissions CELs.
 
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Someone else said find out what caused the code, it's not necessarily the hardware item that triggered the code. I recently fixed an 02 Camry by replacing the air intake hoses from the air filter to the throttle body. They had cracks in them. All the O2 sensor and other codes cleared and the car runs better than ever. There was nothing wrong with anything except cracked intake air hoses.
 
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A genuine high mileage car. What a treat! With no inspection and nothing affected ( performance and economy) I'd just drive it and periodically read codes to make sure nothing new sneaks up. PO420 is a hippie tree hugger code, can't you tell? It's not important smile
 
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