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what's the readings from the oxygen sensors? Use a scanner and post a graph if you can at idle and then with you giving some gas and letting off. Pine hollow on youtube has a bunch of videos on how to properly diagnose vs using the parts cannon.
 

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Like bullwinkle said downstream and it is the front cat (radiator side). It’s probably a cat efficiency issue but once you properly diagnose to determine what the issue is then you have a couple of options.
Check around your cat for leaks for sure.
 

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Discussion Starter · #7 ·
Bill and Tron thanks!
Can I swap the front and the rear sensor on the front and rear warm up three way catalytic converter to check if the sensor is bad? Are the two sensors the same?
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swap the lower sensors from bank 2 to bank 1 and see if the code moves to bank 2 or stays on bank 1. They are the cheaper DOWNstream Sensors. These are AFTER the cat on the underside of the cat before the 3rd cat. the 3rd cat is not monitored so no need to worry about it warming up
 

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Discussion Starter · #9 ·
I haven’t seen P0420 coming back since I created this thread. Whenever the code triggered it’s only when I started the car in the morning. As soon as the engine started the code came on immediately. It sounds like a sleepy or internment bad sensor? It doesn’t make sense it’s a bad CAT because it’s hasn’t reach it’s maximum temperature yet because the car just getting started.
 

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swap the lower sensors from bank 2 to bank 1 and see if the code moves to bank 2 or stays on bank 1. They are the cheaper DOWNstream Sensors. These are AFTER the cat on the underside of the cat before the 3rd cat. the 3rd cat is not monitored so no need to worry about it warming up
I would only buy OE Sensors or at least OE mfg which is usually Denso or NTK
 

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I haven’t seen P0420 coming back since I created this thread. Whenever the code triggered it’s only when I started the car in the morning. As soon as the engine started the code came on immediately. It sounds like a sleepy or internment bad sensor? It doesn’t make sense it’s a bad CAT because it’s hasn’t reach it’s maximum temperature yet because the car just getting started.
It is not a temperature issue with the cat but more of an efficiency of air flow issue.
Cats can throw the P0420 code when the efficiency drops by 5%.
Your ECU/ECM (computer) sees an input reading from the AFR sensor (top sensor on the cat) and from that the computer is looking for a corresponding reading from the O2 sensor (bottom of the cat). If the readings are within certain parameters then that is why the code is generated. I’m not certain what those numbers should be, sorru
People see P0420 and say the cat is bad because that is what usually fixes the problem and therefore you get that advice.,
If there is to much airflow (leak) or not enough airflow (catalyst dirty or cat is starting to clog) it will throw the P0420 code because the computer does not see the numbers it is looking for.
Could be gas related if you are using a new brand/station.
Wife had an 01 Odyssey that threw the cat code whenever she got gas from Sam’s but that was years ago.
Check your gaskets for leaks and all over the cat. A length of hose if you don’t have a mechanics stethoscope will help.
If the cat was easily removable, it is not by the way… you could remove it and try to clean it. It sits vertical so soaking it is not an option unless you get creative. I thought about unbolting mine and shoving a rubber ball or making a block off plate and filling it with seafoam spray and letting it sit overnight, did that with the odyssey when I intermittently got the code later in its life around 300k miles but it was horizontal so I did it in the van.
My MDX was throwing the code so I pulled the AFR sensor and sprayed a can of sea foam spray down in it and let it sit. Be careful if you do this because the seafoam could get sucked back up into your cylinders when cranking the engine. I gas it some gas when I cranked it the next morning outside of the garage of course.
That worked for a while but then I added a couple of spark plug antifoulers to move the sensor further away from the airflow and it’s been fine. I had another issue that caused my problem, oil leaking by the #4 piston ring but that has been corrected.
 

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Discussion Starter · #13 · (Edited)
Tron, that makes sense. So I could have bad intermittent A/F sensor (top of rear CAT) as shown here circled in red or intermittent restricted air flow in the rear exhaust. A lightly clogged rear CAT? The P0420 doesn’t happen too often and if it does the code usually cleared by itself one or two days later.
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YMMV but on our '05 Pilot we got this message off and on and a bottle of Cataclean once a quarter made it go away and was way easier and cheaper than replacing the cat. I kept waiting for it to get bad enough that my solution wasn't enough, but we made it through a few years until we sold it -- kept the P0420 away.
 
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