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Discussion Starter #1
About 2 months ago, I started getting a check engine light in my 2004 Honda CRV which turned out to be a P0420 code. I replaced both O2 sensors and the error went away...until yesterday. Now, the same P0420 code is back again. I'm thinking the next step is to replace the catalytic converter. I can buy an aftermarket part for about $300 and replace it myself. But, I wanted to see if anyone else has had the same experience with a P0420 code. Could there possibly be any other issue that might cause this error code? Thanks!
 

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@purc1234

Hello,
Replace the CAT.

The over simplified explanation on what sets this code is the signal (voltage) coming from the O2 sensor before the CAT should sweep up and down.

The signal (voltage) from the O2 after the CAT should be relatively flat. These signal pattern tells the PCM the CAT is working effectively.

If BOTH O2 sensors signals basically move up and down together, it means the CAT is dead or dying.

See the attached file if you're really wanting to understand the nut and bolts of what sets this code.

I hope this helps,

Cheers!
 

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Discussion Starter #3 (Edited)
Thanks for the excellent response and the the attached document about the P0420 error. After I replaced both O2 sensors, I took a drive to get the CAT up to temperature and connected my scanner to get real-time data from both O2 sensors. The voltage from the downstream O2 sensor was mostly flat at about 0.6 volts (see attached picture). So, I assumed this meant that the CAT was working. But, at 167k miles, it wouldn't surprise me if it was time for a replacement.

I cleared the error code for now and I'll see if it returns a 3rd time. Who knows, maybe I'll get lucky.
 

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Discussion Starter #7
Where did you buy them? A lot of fakes are on the market these days.
I bought them both from Amazon. I paid $127 for the upstream sensor and $80 for the downstream sensor. They weren't cheap (as most fakes generally are) and they had over 500 positive reviews. So, I'm assuming these are genuine.
 

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Thanks for the excellent response and the the attached document about the P0420 error. After I replaced both O2 sensors, I took a drive to get the CAT up to temperature and connected my scanner to get real-time data from both O2 sensors. The voltage from the downstream O2 sensor was mostly flat at about 0.6 volts (see attached picture). So, I assumed this meant that the CAT was working. But, at 167k miles, it wouldn't surprise me if it was time for a replacement.

I cleared the error code for now and I'll see if it returns a 3rd time. Who knows, maybe I'll get lucky.
Please update us with the final resolution 👍🏼
 

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If it sets the code again I would suggest checking the circut again to be sure the O2 sensors are working correctly. also check for other things that can cause cat damage such as excessive oil comsumption. fuel trims being out spec, ST and LT values should both be right about 1.00
If everything checks out then it is time to replace the cat
 

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Discussion Starter #10
If it sets the code again I would suggest checking the circut again to be sure the O2 sensors are working correctly. also check for other things that can cause cat damage such as excessive oil comsumption. fuel trims being out spec, ST and LT values should both be right about 1.00
If everything checks out then it is time to replace the cat
Thanks for the suggestions. After clearing the code, the CEL hasn't come back on again (fingers crossed). I just replaced the VTEC solenoid which was leaking oil down the backside of the engine. Don't know if that might cause the error code. If the error comes back again, I might just go ahead and replace the CAT. The aftermarket part is cheap enough to replace proactively in my opinion.
 

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Discussion Starter #11 (Edited)
I guess my luck ran out. Driving home this evening and the CEL came on again. I pulled codes and the P0420 is back once again! I think it's time to change the CAT.
 

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Discussion Starter #12 (Edited)
I've also noticed a strong gasoline smell around the engine. Could a leaky manifold cause P0420 because the oxygen sensors are receiving extra air through a gasket leak, etc.?
 
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