Upstream O2 sensor question

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You will be fine. The computer will just compensate until the engine is up to temp and everything will work as intended. The engine has certain strategies in its programming to compensate when a sensor is malfunctioning to keep you roadworthy.  

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Both work together. That is why you need to use the same brand and change both if they are aged.  Your fueling is probably leaner that it was  and now running right. You will probably get better fuel economy also.

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I don't believe you will have unbalanced ignition.  I think it does some kind of averaging but nothing too wild. Maybe Cap and shed some light on the strategy it uses but even if they were off by a lot the car would throw a CEL and go into limp mode. When the car goes into limp mode you will not know the difference and it will generally be event free but your gas millage and maybe a bit of responsiveness will suffer but not be dramatically apparent.   

When your fueling is correct everything else will fall in line and the car will run as intended. It can also change the way the car sounds.   02 sensors in many ways is a very important sensor because it is the final arbitrator of how the car should run and if it drifts or switches slowly the car will compensate but it will not be at its peak. 

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It takes a few drive cycles before you can see where the LTFTs settle.  If they come down from where it was at it is a good indication that the the previous  02 sensors  have drifted as they are designed to.  Running rich is much safer than running lean however running rich is not good either as you can develop deposits and fuel wash increasing wear however that is all programed in to the ECU as time goes on so it's forgiving.  Most people don't maintain their car to keep their aged engine in top condition so it performs well because they have been acclimatized to waiting for a fault and even then there are many that drive with a CEL all the time.  

As far as the rear post cat 02 sensor goes you can use any old one with an intact heater. What I generally do is keep an old 02 sensor just for this purpose because all this 02 sensor does is check to see if the cat is operational and nothing else.  

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Posted by: @nottamiata

Wish I had known that before I cut the wire down to the quick on the good one I removed yesterday. But, at 80K miles and $30 for a new sensor anyway, its probably not worth fussing over it just tl have it die in a month. <shrug> Good to have in a pinch though, and the knowledge is *always* appreciated.

It still astounds me that the connector adds $30 to the part. I guess they have gold, palladium, and little rubies in there? No? You mean it's just simple price-gouging for a fifty-cent connector? Say it ain't so!

In the real world it doesn't work like that. It is the rarity of the part that commands the price of keeping it on the shelfs.  Actually it is averaged with other parts so in effect if it wasn't averaged we probably would pay more for parts like this. 

Of course we can make our own on the cheap by  splicing the wires with the universal kit but at that point its labor.  If you give a mechanic a universal kit and asked how much it would be to splice it in with the original harness it would be more than the ready made  part.  

This is where DIY can save you thousands of dollar bills. Having the tenacity of doing it yourself and not  depending on someone else can save you a small fortune but its more than that as there are those of us who actually like the labor of love.  

Then there is an intersection of using your brains and knowing that although you can you shouldn't and it should be done by a qualified person otherwise you may risk injury or death.  

The best way to know all this is to take a balanced approach and lean the fundamentals, buy the right tools and above all else read the instructions.  

About 15 years ago before the universal kits were available it would cost a fortune for replacement 02 sensors in the range of $120-150 for each one. Now they are cheap. 

Posted by: @nottamiata

How does fuel wash affect a catalytic converter, particularly one very close to the exhaust port?

I am inclined to believe, while admittedly still very ignorant, that the Spyder precat problem was caused by burning oil. But then why do facelift cats also occasionally disintegrate? More than one cause, obviously. I am being careful not to make an assertion, just a topic for discussion. Or is this one just way too tired?

Probably not enough to matter. That little bit of enrichment should not cause any harm to the cat.  The Spyder precat issue is believed to have multiple causes but nothing definitive.  Just one of those things where you stop wondering and just get them out of there not to worry about them. 

As far as a slightly rich mixture over a long period of time it could cause deposit  formation, dilution of the oils that lubricate the cyclinders and turning the oil acidic earlier but not enough to make a difference if you use quality oil as they have buffers.   

You probably won't notice anything as its a very slow process that will be apparent if you compared two aged engines.