Steve - Lots of good info, and a good thread. It's also interesting because this is not as simple as it seems. Let me preface my comments by saying that I am not the expert here. I can offer some rational maybe, but might be about the limit.
1. I don't know this, but it would seem that whichever cat has a sensor before AND after it, would be the main cat. Which makes me wonder why the 2nd cat needs to be there. Which also makes me wonder, what my car would sound like without the 2nd cat. I want it just a touch louder, but not loud. It's possible I could yank off the 2nd cat and not get a check engine light, and sound like an 8V72? I'll have to think more about this later.
2. O2 spacer. I've had some experience with this. I bought a $50 "minicat" from something like Big Daddies (seems like). And it did nothing. I still got the check engine light. This was on my son's RX8 with a failed cat (that was removed and replaced with a double resonator straight pipe). The RX8s tend to go through cats, and the added heat on the wankel due to a cat being there (especially one that is bad) causes Wankels to go bad too. So we decided to try to just delete the cat. Anywho, since the first crap mini-cat wasn't working, I found a <$10 one on Walmart and decided to give it a try because it was so cheap. The $10 worked like a friggin charm. He's put thousands of miles on the car with that cheap mini-cat on it and the CEL & code has never returned.
If my car were having the problem you are having with your car, I'd be very tempted to try to run the car catless (if the cat cleaning didn't work), but with the mini-cat. I would have warranty issues. However, the way the Magnusson-Moss Act reads is, I would be losing warranty only on the exhaust portion of my drivetrain warranty. I could care less about the cat, but I would rather retain an unquestionable warranty for the duration. Of course now that I think about it, I doubt the exhaust is covered under the drivetrain warranty anyway.
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View my fuel log 2020 Mirage ES 1.2 manual: 42.5 mpg (US) ... 18.1 km/L ... 5.5 L/100 km ... 51.0 mpg (Imp)