For what it's worth, it's not uncommon to see a TWC (Three-Way Catalytic Converter) degrade/begin to fail after 150k after normal use. That's not to say they can't make it over 200k without issues, just that there's a lot of variables at play and seeing one set a DTC for efficiency at 180k is not surprising or out of character from what I have seen.

The main way the system detects the efficiency of the TWC is by checking it's ability to store oxygen by using the AFR & O2 Sensors, as well as it's own internal calculations based off some other ECM inputs (that's something I don't think you can really see the ECM doing, only the result of what it comes up with.)

I just ran into this with an old 2nd gen Prius that had something over 200k, it came in originally with just a P0171 (System Too Lean- Bank 1). Looking at the MAF Sensor (useful to look at on many vehicles with similar issues, though on a hybrid it's a bit of a pain), AFR Sensor, O2 Sensor, and Fuel Trims; it was running too lean (something like 28% Long Term Fuel Trim); vehicle ran well for an old Prius and the customer hadn't noticed anything too weird beyond the MIL. Inspected for any intake leaks and exhaust leaks and didn't find anything, did find the MAF Sensor to be rather dirty and have some fluff on it (likely from someone not clearing the air box properly before throwing a new filter in it), cleaned the throttle body and any residual oil in the intake as a courtesy. Cleared DTCs, set vehicle in Check Mode (One-Trip Detection Mode) and gave her a quick test drive, within two miles LTFT dropped from 28% to just 4~6% (we like to see +/- 10% in general for a normal vehicle). Told the customer to have a nice day and keep an eye on the vehicle for any reoccurrence/odd behavior.

They came back a week later with only a P0420 (Catalyst Below Efficiency- Bank 1), not entirely surprised with that outcome; but wait, why didn't it set a DTC the first time it came in you might ask? Well, because the P0171 beat the P0420 to the punch; the moment the DTC got set it stopped monitoring the Catalyst and thus it never bothered to notice the catalyst failed its monitor. So we start with the usual stuff, verified MAF, AFR, and O2 sensors are functioning as designed, our fuel trims were right where we wanted them to be; so now we're most likely looking at a TWC failure (possibly due to age, possibly due to running lean too long, it doesn't matter at this point on a 2008 Prius with 200k+ miles). Now before recommending a catalytic converter and calling it a day I did have some minor frustration with the AFR and O2. The sampling rate on a TIS Techstream through the OBDII connection leaves a bit to be desired in general, particularly on older vehicles; but in this case I wasn't particularly happy with how long the AFR and O2 sensors took to warm up and start signaling properly, they were a bit too sluggish even by the FSM's standards (though oddly did not set any DTCs) and the signals looked "okay-ish" but still slightly off.

So at this point from a gut/experience perspective I'm leaning on a TWC and two sensors to both be safe and ensure there isn't a 3rd visit from this customer. To verify my gut feeling, I should have looked at the prior test result from the Catalyst Monitor; but monitor results get forgotten/overlooked by a lot of technicians (myself included) because we can usually see real-time Data List that will point us in the right direction. But in this case since I was a bit skeptical of the sensors I went ahead and cleared the DTCs, set the vehicle in Check Mode (this is a really useful feature) and test drove it for about 10mi while watching the Data List. All sensor readings, fuel trims, and such were relatively good; though again, the sensors were still a little "off" until they fully warmed up. After the test drive the Catalyst Monitor reported "Passed" with a value of ".032"

I can't really tell you what .032 is being measured in (ppm, g, or maybe bushels lol?), but it doesn't matter. What matters is that the Monitor reported: Result- Passed, Measured- .032, Min- .031, Max- .999; this tells us we have what is often referred to as a "Borderline Catalyst" one that is just on the failure threshold and will report good some days and bad other days. Now that combined with the other information we've gathered makes me more confident in recommending a TWC replacement because it's failed/failing, as well as a new AFR & O2 Sensor due to age/mileage/degradation affecting their responsiveness/efficiency. It's an expensive repair for a customer, but I am fairly confident with my recommendations. Customer approved the repairs and the new sensors are much better as warming up and giving more "stable" readings, new Catalyst passed it's monitor with flying colours.

Now the reason I even bothered to post all this pedantic story is to show how annoying and generally boring Catalyst failures can be. You do enough of these types of diagnosis and repair in the industry long enough and you start to see a consistent pattern that you can rely on 9/10. Only a P0420/P0430 DTC is set on an older (150K, 10yr+) vehicle, MAF, AFR, and O2 sensors seem to be reading well; Fuel Trims look relatively okay, etc. then it's the catalyst. That's a "Gut Feeling/Experience Based" diagnosis that most experienced techs can pop out in under 10min from just the FFD (Freeze Frame Data) and/or a quick peek at the Data List; most of the time you don't even need to double check anything. In the case of the Prius, I could possibly have omitted the sensor replacements, but I have come across a handful of instance on fairly old vehicles where we replaced the catalyst and one or more sensors trip a DTC for something that ends up being caused by a sluggish/degrading sensor; so these days I will usually at the very least throw a rear O2 sensor in with a Catalyst Replacement out of habit (in the case of this Prius the readings were slightly weird so I dug further to verify that it wasn't possibly just the sensors being weird causing the DTC).

So take the above however you want, you can say I and other seasoned technicians are lazy because we see that DTC and a high mileage vehicle and just go "you need Cats" without explaining further and the whole above post is just justification for our laziness. Or you can see how someone forms a "Gut Feeling/Intuition" by seeing many of these similar cases and learning what information is important/worthwhile and what isn't to trim down their diagnostic time and better serve the customer and reduce the amount of times they have to come back for repairs and/or labor their charged (i.e. in the case of the Prius, I just rolled sensor replacement into .5hrs for both since I was already replacing the catalyst; the .5hrs actually worked in that customer's favor; I had to pull the exhaust manifold out because the B1S1 AFR Sensor stripped out the bung coming out despite my best efforts so I had to remove the manifold to try and chase it and do my best to save the manifold for the customer rather than just recommending a new one since they were so nice to trust us with such an expensive repair.) Or we could just charge the customer diagnostic time + repair for each visit they do and rack up billable hours (usually charge at least 1hr flat diagnosis fee and 1hr flat sensor replacement fee unless it's not easy to get to; so you can see where that adds up fast for a customer.)

now that we are talking CATS, i just got a p0420 code. is this cat on the passenger side for p0420? checked for leaks and found a leak between the first cat and the second cat. there's a little bit of leak at the exhaust manifold.

i agree, when doing this expensive repair, it's best to replace the af/o sensor at the same time. i think i have a ptsd working with exhaust. lol!

by the way, i have 155000 miles now.

Had an P0430 code pop up. Cleared many time and the code pop's up again pretty quickly. Never looked at O2 sensor data. Had a Denso O2 sensor sitting around so I figure to put it in. It has about 60 miles on it. I can not get the P0430 code to come on. Will update after a few hundred miles more.

At about 310,000 miles i started throwing p0420 and then shortly thereafter p0430....i tried Cat Clean, lacquer thinner, looking for exhaust leaks, replacing a downstream O2, hoping, praying, cussing, bargaining with God, watching nearly every relevant YT video, hours of techstream data dives, etc. And then bought some Canadian manufactured aftermarket cats, installed them with new hardware, gaskets, and replaced both A/F and the remaining original O2 sensor , all with new Densos...

Joy lasted about 10,000 miles then P0420 and P0430 came back. Tech stream each time showed way too much activity in the downstream sensors and the temperatures were all wrong pre vs post cats.....finally bought factory cats (495 each) and did the installs myself. NOW techstream shows proper o2 system function.....and i am at 349,742 miles.

Moral is, trust the numbers and data, Cats DO wear out. If you need replacments, just do factory ones and dont look back.