P0420

Looks to be the vacuum valve for the EVAP. My opinion is it's probably fine if not throwing P044x. You can blow both ways thru it and verify the check valve working. Your ST fuel trim numbers don't look bad to me given the age of the car, and you may be able to get the "high" down some with a simple MAF/throttle body cleaning. Vacuum leaks are not common. You could do typical testing right on up to smoking it, but I'd expect P0171 if a lean condition was present.

The P0420 is almost always the cat with gen 2 Prius, and Toyota in general. Personal experience with the brand over many years is that the the A/F and rear O2 sensors age in unison, and will go long haul without coding. Then once you replace the cat, it's not long before one or both sensors throw a direct sensor circuit code. Hope this helps, although I suspect it's not really what you want to hear. Good luck with it.

 

P0420 is a "cat efficiency" code, but as I have posted before, Toyota and Subaru have VERY LOW thresholds for setting it, WAY lower than almost all other OEMs and far lower than has any emissions impacts.

Your AFR (lambda) is 0.994 is only 0.06% rich (1.000 = 14.7, lower = rich, higher = lean; deviation of air from "ideal").

There are two DIFFERENT oxygen sensors in these (and most other) cars, the front is a "wide band" sensor and the rear (downstream) is a "narrow band" sensor. The ECM uses the upstream sensor for air/fuel control and the downstream to monitor aftertreatment (catalytic converter) function. Anything that causes the upstream sensor to read incorrectly (exhaust leak, dirty MAF sensor, etc) will cause the ECM to incorrectly calculate the cat efficiency -> Garbage in = garbage out.

About the only way a cat on these cars goes BAD, is from ingesting oil from a badly worn engine or one that was severely overfilled at a service event.

 

I'm new to posting, usually just read. My 2005 Prius with 242700mi. keeps giving me an intermittent P0420. The common denominator is it only happens after I fill the tank with gas. Which leads me to question. Does the prius have an EPA can for the air inlet to the fuel tank that is connected to the air fuel mix/o2 EPA sensor garbage that could throw a P420 code?

 

Haha, here is an interesting story. I have been getting a "P0420" error engine warning lite. (catalytic converter might need replacing).
Replaced the spark plugs, both O2 sensors, next step was to look for exhaust leaks before just adding new 3rd party catalytic converter.
Well, I found a reliable muffler guy thru NextDoor website and they did some research on my car (5 minutes) and told me that the engine mounts were old and when the engine was revved, it was creating a leak where the exhaust was connected to the engine.
This also explains why the light was intermittent, and I could turn it off and it would stay off on long trips: ergo: no engine torgue from fast starts.