Could also be caused by the post cat O2 sensor being defective. You need to read the outputs of the O2 sensors with a decent scanner capable of reading the voltages of each. No other way to check.
It it's not the post cat sensor, you are, as Rhotpursuit says, pretty much in need of a cat. You can drive it awhile as is, if there are no driveability issues. Eventually it will cause problems.
The only one I've ever seen that had a P0420 and a bad O2 sensor had a bad upstream sensor (very slow, but just under the threshold for setting a "slow" code). The upstream was so slow that the downstream had nearly the same waveform, tripping the P0420. I'm not sure exactly how a bad downstream O2 (with a good upstream one) could trip a P0420 without tripping another DTC that's related to the O2 sensor.
Other than the one time I've seen a bad upstream cause a P0420, every other P0420 I've seen needed a catalytic converter. I've probably averaged 2 to 3 per year over the past 25 years or so. That's around 1 every 18 months for the past few years when I've been semi-retired, and as high as one every month or two when I worked in busy shops and dealerships.
At 177K the cat, if it's the original, is likely past it's operational lifespan or as stated above contaminated with something.
If you are even thinking about a bottle of catalytic converter cleaner, be sure to get it from a snake oil salesman on a covered wagon.
It's still the same formula it was in the 1860's. 2% water, 49% gullibility, and 49% ignorance.
There will always be somebody willing to take advantage of people looking for a simple cure.
I'm not going to recommend snake oil. However, I've seen some success with "cataclean" (against my recommendations) after I fixed the coolant leak issues that poisoned their catalytic converters, and both instances that I've observed successfully cleared the P0420 at least for a couple years. One of them ran for more than 5 years and never had another issue with the cat. That owner brought two bottles to me, "Cataclean" and one other product and said he understood that if it screwed up his injectors, fuel pump or anything else fuel related that I was going to charge him full rate, full labor for the fix. He said it wasn't a question of whether he was going to use the stuff, the only answer he wanted from me was which of the two products he had was he going to pour in the tank and which one was he going to return. I picked the "cataclean" as the least likely to be harmful.
The other one, I'm not sure what they used, but they came back almost 3 years later with the P0420 again, and I was starting to look up the paperwork for a warranty claim on the cat when I realized that I never changed it out. They said they used "some cleaner in the fuel tank" to "fix" it after I did the gasket job when it had the P0420 error. Whatever they used, it cleaned it up enough to work without the P0420 code for almost 3 years.
That stuff is firmly in the "can't hurt much, might help" category with me now. I wouldn't recommend it to anyone, but if you want to try, it's probably not going to hurt anything other than the cat, which is already going to need replacing if the "cleaner" doesn't work.
The cat in my 2002 tutu was damaged by a misfire. The spark plug anti- foul trick worked until it started to rattle. I bought a cat from Amazon for $110 that included bolts and gaskets https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
My 1992 has the original cat @ 357k
One of the instructors at a GM school said "Catalytic Converters never die of old age, they are always murdered."
I would recommend the OP spend a little money on the BAFX OBD-II adapter so they can do more diagnostics, and a spark plug anti-fouler. That's more likely to "work" as a temporary fix than a new O2 sensor. And having the diagnostic tool to check on things will help determine when the problem is back and bad enough to warrant a new cat.
And I completely agree about cats not dying of old age. They are always killed by something else.