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Discussion Starter #1
The CEL shows up on my 2005 MDX. I did a scan with a simple scanner I bought. Got a generic code P0420 with the message: "Catalyst System: Efficiency Below Threshold --> Bank 1".

I understand it's most likely due to a failed O2 sensor. Could be due to the cat converter. But I think it makes more sense to change the cheaper part first.

What I don't know is if I should change the upstream sensor or the downstream one? Can someone advise? And where exactly is it located?

Thanks.
 

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No, it's not likely the O2 sensor. What it's telling you is the cat isn't cleaning up the exhaust enough to meet the parameters set for the ECU. This is a very common issue for older/high mile MDXs. In most cases the cat is still flowing fine, just not cleaning the exhaust enough. I don't have to pass smog so I just did the non-fouler hack and my light has been off for 9K miles and it runs just fine. If you have to pass smog testing and/or want it done "right" then replace the cat and both sensors while you have it out. Some have reported doing the non-fouler hack and still being able to pass a smog test. Remember, the local smog regulations can vary as well as the calibration on the sniffer, so it's possible for the car to be polluting "too much" by the ECU standards, but still be legal by local smog testing thresholds. If you have to smog test though they will never pass a car with a CEL on.
 

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Got the same code in our 06 years ago. Factory manual states bad quality gas can also cause the code so first tried HEET water remover, Techron, and 91 octane gas from Shell. That didn’t resolve it so changed the Cat. Bank 1 is near the firewall. DIY with a factory cat and new gaskets at the head and exhaust. Not hard but just tedious to get the heat shield off and out of the way and limited space. All the nuts and bolts were pretty clean so a little bit of PB Blaster penetrant worked fine. Worked from the passenger side with the front wheel off.

Good luck!
 

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The problem could certainly be a bad cat, but I'd seriously suggest swapping out the post-cat (downstream) sensor, since they do fail regularly, and are a whole lot cheaper and easier to swap. Worst case, you have to swap out the cat anyway, but are now starting with a new O2 sensor (which is really just a maintenance item, much like a spark plug).

Also, if the problem occurs after some sort of misfiring or oil burning event (one that's been fixed, hopefully), then you might try running a can of the cat-cleaning snake oil through your gas tank. My sister's Subaru was throwing the same code, and some highway miles with the snake oil seems to have fixed the issue. Again, not nearly a 100% sure fix, but I like going with "cheap and easy" if there's a reasonable chance it'll work.

Either way, let us know the outcome... anecdotal info is pretty valuable in a forum like this, since it accumulates over the years and will give future searchers a better view of reality.
 

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Discussion Starter #5
No, it's not likely the O2 sensor. What it's telling you is the cat isn't cleaning up the exhaust enough to meet the parameters set for the ECU. This is a very common issue for older/high mile MDXs. In most cases the cat is still flowing fine, just not cleaning the exhaust enough. I don't have to pass smog so I just did the non-fouler hack and my light has been off for 9K miles and it runs just fine. If you have to pass smog testing and/or want it done "right" then replace the cat and both sensors while you have it out. Some have reported doing the non-fouler hack and still being able to pass a smog test. Remember, the local smog regulations can vary as well as the calibration on the sniffer, so it's possible for the car to be polluting "too much" by the ECU standards, but still be legal by local smog testing thresholds. If you have to smog test though they will never pass a car with a CEL on.
Where I live, I don't have to pass the smog test either. Could you let me know what "non-fouler hack" you refer to so that I can try? It's great that it gets the CEL off but when you keep driving with a faulty cat like that, doesn't it hurt the car's performance such as weaker acceleration power, mileage efficiency, etc?
 

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Got the same code in our 06 years ago. Factory manual states bad quality gas can also cause the code so first tried HEET water remover, Techron, and 91 octane gas from Shell. That didn’t resolve it so changed the Cat. Bank 1 is near the firewall. DIY with a factory cat and new gaskets at the head and exhaust. Not hard but just tedious to get the heat shield off and out of the way and limited space. All the nuts and bolts were pretty clean so a little bit of PB Blaster penetrant worked fine. Worked from the passenger side with the front wheel off.

Good luck!
I already used 1 bottle of the Techron Concentrate Plus Fuel System Cleaner if that's what you're referring to. I'll try 1 more to see. Do you need to weld during the installation of the new cat? Cat Bank 1 is the one in middle of the car, right, where driver & passenger seats are? If it is, then I think I had it replaced 2 years ago, with an aftermarket one.
 

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I already used 1 bottle of the Techron Concentrate Plus Fuel System Cleaner if that's what you're referring to. I'll try 1 more to see. Do you need to weld during the installation of the new cat? Cat Bank 1 is the one in middle of the car, right, where driver & passenger seats are? If it is, then I think I had it replaced 2 years ago, with an aftermarket one.
With that additional info of the cat being replaced before I would definitely suspect the cat has already failed. Aftermarket cats are famous for 1-2 years max, but it's not uncommon to get a 5 year warranty with them. You should definitely check to see if there is. Yes, P0420 is for downstream bank one, which is the firewall side.

The non-fouler hack is taking two non-fouler spacers that are actually made for spark plugs, but the threads on an O2 sensor are the same. You drill out the one spacer and then stack them, remove the O2 sensor, screw in the stacked non-fouler spacers, then screw the O2 sensor into the stacked spacers. Youtube it to see it. Really easy to do. It basically makes the O2 sensor less sensitive by taking it out of the direct exhaust stream. Cost is about $5 and your time. Hand tools and a drill bit is all it takes.
 
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