[maxud]

The O'Reilly's tool was a loan-for-free (refundable deposit), not rent-for-fee. Since I'm not going to buy the sensors from O'Reillys, maybe I'll just buy a tool from them. I've got a $5 off coupon I keep forgetting to print and use.

All auto parts stores Advance, Autozone, O'Reilly use the same loan system. You pay for the cost of the tool and if you return it within certain period, they refund the entire cost and tax making it essentially free. Of course they will not let you walk out with the tool on a promise that you will return it.
You can rent tools regardless whether or not you are buying the parts, but of course they would like you to.

 

[maxud]

Is that a universal size for any O2 sensor? When I changed the one, O'Reilly's offered me a loan of one of those, but maybe I should just buy one.

EDIT: looks like it...

nothing against buying, but if you do, invest in quality tool and not off brand chinese junk that will split the first time you go to use it.
The rental tool will likely be of higher quality than ebay knock off.

Consider OTC, KD, Gearwrench brands.

 

[maxud]

Is that a universal size for any O2 sensor? When I changed the one, O'Reilly's offered me a loan of one of those, but maybe I should just buy one.

EDIT: looks like it...

I have this tool kit and typically find the socket that works
https://www.nationaltoolwarehouse.c...RCYARIsACEBhDNWEcrtaLsRoqUTEIS278d7NjMXbeQ4v9itbF1OG7IkTSLTkqCiocgaAvXzEALw_wcB

but a lot of them are 7/8"

 

[maxud]

I've had pretty good luck with this style O2 sensor tool:

The above hasn't yet failed me. I also have the slotted deep well style socket and it doesn't seem to work as well. I've noticed it likes to flex too much. Cutting the wires and using a box wrench is also a good method but once you do that you're definitely committed.

I can offer suggestions on what I have done before, but few people would go to such extremes. I actually took apart the pig tail, removing the pins from the plug. This way,
you have a plan B if removal fails.
My experience on both shallow and deep o2 sensor socket tools is that if the sensor is stuck, the socket flexes but does not budge the sensor

 

[maxud]

Thanks, this is all very helpful.

I don't remember what kind of sensor I put in, I'm pretty sure it was a cheaper one, not an OEM one. Bosch maybe. If I call O'Reilly I think they can look up my purchase history from my phone number and tell me. I guess this is an opportunity to buy myself a crow's-foot as well.

If I end up having to replace the bank 1 cat, since OEM seems to be the only reasonable option, I guess I'd have to get that from the dealer.

There are special tools for removing oxygen sensors, so I suggest you look into that, but personally I have found that box wrench (closed loop) works best for taking off
old stuck sensors. Especially if you are replacing it, you can just cut off the pig tail. Alternatively, deep socket also works but depending on the location.

There are a ton of online honda dealers that sell parts at 25-30% off msrp. The trick is of course to buy online, vs going to physical parts dept at the local dealer.
I like BernardiParts.com on east coast but have bought from many different ones. Yes only buy OE Honda catalytic converter. Not only will it provide perfect fit,
it should last as long as original one did, provided, as John mentioned, you get at the root problem that caused it to fail, so definitely good recommendation to replace
both pre and post o2 sensors.

 

[maxud]

That's what I was thinking; when I replaced the forward O2 sensor it was hard enough to get at when I could see it, and I could barely reach&feel the rear one, I was thinking no way I could do this blind! But maybe there's a way to prop myself up on top of the engine so I can see behind there?

So I asked a buddy if he could maybe help me with the cats, he's a lot more experienced than I am with auto repair -- but he said as far as he knew cats always had to be welded on/off. Is he just used to older cars? Gen2 looks like just bolts, and it sounds like Gen3 as well.

Yeah, you can prop yourself on top of the engine, get ready to be extremely uncomfortable.
Make sure you have a full spectrum of flexible ratchets and wrenches.
Those O2 sensor sockets are wicked on Honda's, some times requiring removal of bracket before then can be removed.
Whatever you decide to do, make sure you have a plan B when plan A goes to hell.
If you are buying Honda OE cat, it will bolt on perfect to existing flanges.

 

[maxud]

Are you sure that's right? It would make more sense to me if bank 1 is front and bank 2 is rear. Isn't that the order of flow, front to rear (1 to 2)?

lol, are you serious? What flow are you talking about? air flow from front of the car?
yes rear bank is bank 1 (cylinders 1,2,3), front bank is bank 2 (cylinders 4,5,6).
When you are working with cars, its not about what makes more sense, its about specs.
Ody's engine is transverse mounted, so bank 1 is the bank where cylinder 1 is, which in most transverse mounted engines will be rear bank.

 

[maxud]

I have never done a rear cat on ody, but I imagine that it will be relatively hard job to do. The rear bank access will be mostly blind, reaching over the entire engine to undo 4 nuts that hold the top of the cat to exhaust manifold. The bottom 3 bolts or nuts are not too bad, as long as you got an impact tool and the fasteners are relatively rust free.
But one broken stud will make your life miserable. You wont know until you try.
If possible, just disconnect the o2 sensor harness and leave them in, it will be much easier to remove the sensors on the bench.
Likewise, install the o2 sensors before installing replacement cat. Use new gaskets.