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The upstream O2 sensor, it seems, trims the mixture... The downstream O2 sensor appears to check that the converter is operating properly.
Correct. Those are the functions of the 2 oxygen sensors.

All the heat shielding and wrapping of piping should help heat up the catalytic converter, and it's not very far downstream anyway.
 

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So, in essence, if there are not three O2 monitors, the (often-times larger) underfloor cat, goes unmonitored by way of checking if is poisoned or crumbling or...?
 

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Correct. Those are the functions of the 2 oxygen sensors.

All the heat shielding and wrapping of piping should help heat up the catalytic converter, and it's not very far downstream anyway.
Did you not see the pic i posted? Its pretty far away lol.
So, in essence, if there are not three O2 monitors, the (often-times larger) underfloor cat, goes unmonitored by way of checking if is poisoned or crumbling or...?
Im not so sure how much it does, cause the exhaust only smells slightly different and ONLY during the cold start up where its dumping flames down the pipe. After that, it smells like i had it still on there.
 

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So, in essence, if there are not three O2 monitors, the (often-times larger) underfloor cat, goes unmonitored by way of checking if is poisoned or crumbling or...?
Correct. I didn't see anything in an ultra-quick (meaning I really didn't try) search to see whether both catalytic converters are 3-way converters or not. It used to be somewhat common to use a pair of cats, neither of which was a 3-way catalyst. I've no idea about now. Either they're only required to monitor the first catalyst, or it's the one using up the free oxygen, or something.
 
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What with the rash of catalytic converter thefts in North America I was wondering how vulnerable our third gen cars are. Also, is there a preferred means of safeguarding against this? I have heard of "CatStrap", along with an optical (?) alarm (with logic). Also, other cable-type affairs, sheet metal aux. panels, etc.

Tilt alarm?

What would frost me as much as anything would be indiscriminate rocker panel jacking damage and other general destruction involved in actually getting to it. I have insurance but don't want to go there at all.

Ideas, comments?
Doing some reading I found that it's pickup trucks and higher off the ground SUVs that are commonly the victims of this theft which makes sense for their easier access. Thieves learn which vehicles are easy pray for what ever they are after and now it seems cat's are the in thing to steal.
Low to the ground sedans like a Mazda 6 are probably safe from this theft. As described by knowledgeable people here, Mazda seems to install their cat's to where they are hard to remove, at least quickly so I wouldn't lose any sleep over it owning a Mazda 6. Any theft is usually hit and run to reduce the chance of being caught.
Below a segment from an Internet article on what are the most commonly stolen.

The most commonly hit vehicles are SUVs and trucks, especially late-model Toyotas, because they sit higher off the ground (making for easier access) and the bolts that connect the converter are easily removed. More Nissans are being targeted as well.
 
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