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Discussion Starter #1
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My engine is throwing a P420 code (catalyst System Efficiency Below Threshold (Bank 1)) for over a month now. I got a brand new rear O2 sensor and drove 400 miles mixed highway and stop go with no light and no issues at all. After about a week the light popped back up and P420 showed up again. The engine runs smooth, idle is rock solid, has great power and doesnt give any signs of a bad cat (that i can tell). Any Ideas??
 

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Nothing sounds wrong, other than your PCM is telling you that your cat is inefficient. You can and should do some tests to confirm this. Monitor 02's with scan tool -and- check cat temps with IR Temp Gun. This video will show and explain the deal.

https://www.youtube.com/watch?v=9VZ5K8n5jj0

Confirming cat is bad is rather easy; finding out why it failed (outside of old age) can be harder/tricky to figure out.

As you likely know, once you have a P0420, your kinda screwed if in a State that require emissions testing.

Edit: A car can run perfect and still have a bad cat.
 

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Discussion Starter #3
My state does have emissions testing so thats going to suck in 7 months when Im due for inspection. Its strange though because when i reset the code I checked after all the driving that I got past the diagnostics with my scanner. Everything was OK and the "drive cycle" or whatever was complete and then a few days later the code came back. I can actually sneak by emissions if I can repeat this.
 

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I can actually sneak by emissions if I can repeat this.
Yes, IF you can repeat that. As time goes by, that window may get smaller. Sounds like you know, but I will say it anyway, after a reset the PCM will hold on to the P1000 (emissions/monitors incomplete) for several days and during this, you will also be unable to pass emissions. Emissions here in AZ told me to take a hike and come back after my P1000 was gone. Dumb me, had car battery disconnected just days before.
 

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Small chance in hell... but a new/different upstream O2 sensor could pacify it just enuf to prevent the P0420... might also make the P0420 come on quicker.
 

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Discussion Starter #6
Okay so a little more back story: I had a head gasket (I think) and thermostat housing blow on my 170K motor back in september and though the forums here i found a motor with a little less than 100k miles on it and swaped it in myself. Im thinking that the blown head gasket, all the coolant that got thrown into the cylinders/ oil ect. could have done some damage to my cat. I assume a lot of unburn fuel went into it. Since swapping the new motor in everything has been A OK except the P420 code.

Does this make sense? What are my options living in a state with emissions laws?
 

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Discussion Starter #8
The rear O2 sensor got changed a few weeks ago =(
 

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Discussion Starter #9
The good news is it is NYS so all I need to do is pass the scanner test. So anyone know what some good aftermarket cats are?
 
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