a Famous P0420 Code

This was my experience even on a '98 CRV

OP, if you do a non-fouler, don't do just one. Stack at least 2 together, and I'd even consider 3 if there's room.

I'll have to disagree with you on this one. While I've used this trick in the past, I found that using more than one of the "long reach" non-foulers almost guaranteed a new code: B1S2 Low Activity. The PCM needs to see SOME action by the rear O2 in order to fulfill it's logic checks. I experienced this as far back as my '98 Seville, so it doesn't just affect newer vehicles.
 
I erased the code last night and after 24hrs and nearly 200miles it looks like this; ill do the readings in the morning again

before that last night I removed the fuel cap and reset it, and pour a bottle of Gumout injectors cleaner at 1/2 gas tank to see what happens

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Either the cat is bad, or the O2 sensors are weak, and those bad (inaccurate) readings result in the cat efficiency code.

Drive it hard for a bit. Check the codes. Then, OEM O2 sensors.

O2 sensors usually don’t last much past 100,000 anyway. My $$ is on a weak sensor.
 
thank you; that's what I'm doing it right now

I was wondering, if catalog is bad or the O2 sensor which one goes out first normally; for example the first catalog converter or a sensor, the first in the line from the exhaust manifold perhaps?
 
It’s been 50/50 for me. I would go with the upstream sensor first - since it controls engine efficiency. If it’s worn out, you’ll get better MPG/performance with a new one.

The downstream one is there solely to measure cat efficiency. Hence the advice to simply install a spacer.

I’ve had to do a spacer with a high performance cat/downpipe, even with a new downstream sensor.

So, if the upstream doesn’t resolve it, I’m not opposed to the downstream sensor in principle. That said, if it were mine, I would go with a new downstream sensor if the upstream didn’t fix this, but I am super picky about things, so…
 
i'll be patient then; right now it looks normal, it does use extra gas i'd say but no light is back on; more of hwy miles i'll do in coming days; I just register it so we are good for 12 months

also over the weekend ill visualize the exhaust system; the first thing when wife said something is wrong with the gauge cluster, id ask her whether she hit a pothole or ran over something so it could mess up the exhaust system but she said no

so let se in coming days how the system will react
 
thank you; that's what I'm doing it right now

I was wondering, if catalog is bad or the O2 sensor which one goes out first normally; for example the first catalog converter or a sensor, the first in the line from the exhaust manifold perhaps?
You should really have a mechanic troubleshoot the car, could just be an air leak or some loose component. It's either the O2 sensor or the catalytic converter. That code is typically the catalytic converter because it means the downstream sensor is reading the same as the upstream sensor which means the catalytic converter isn't working. Now the reason they're reading the same could be an air leak somewhere or maybe there's something wrong with the upstream. The upstream one is the one that normally goes bad first. At that mileage, even if you had no codes, putting in a new one would probably improve gas mileage because the switching gets lazy as they age so a new one would give you slightly better gas mileage and probably pay for itself over the next 100k. Doesn't look like they're that much, the upstream Denso is about $95 on rockauto but I suppose you could try a cheaper one but I don't think you save much at $88.
 
an update

So after 4 days and about 400 miles the check engine light did not come back; the OBD IM feature does not register anything wrong with the car;

Visual inspection does not confirm anything out of ordinary in plain sight or sound; honestly I don’t thing the check engine light it come back

However, this take shows just how sensitive is the car and I’ll stick to M1 oil only; tomorrow I’ll change spark plugs and PCV valve just as a measure of preventive care; I also think in coming years I’ll need to replace the catalic and sensors too anyway because they all do just not last nowadays as back in the day

In the end owning and running a car is such a lottery, you just never know

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I'd say you're in the clear. So many outside factors could have caused the initial fault that it's almost not worth addressing. Heck, a lousy tank of gas could have cause just enough of a change in combustion efficiency to set a code. Glad to hear it worked itself out.
 
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