P0420 code help

It appears that you are reporting 33-36 mp[g] when driving. Compared to my 2006 Prius, this is extremely low gas mileage. Gas mileage is important because a number of conditions often provide their first clue as decreased gas mileage. One almost certain way to destroy a catalyst is for the catalyst to continually ingest oil from a badly worn engine*. Low gas mileage is a clue to the engine being a worn-out oil burner**.

See posts #14 and #21 from the following thread: Oil Catch Can from Ebay Installed | PriusChat

Can you let us all know the history of your Prius and oil consumption (as it may help solve your P0420 issue)?

Can you monitor what the o2 sensors are reading and let us know the results?

However, in a different Prius Chat thread, one participant asserted that his P0420 issues were caused by worn/bad motor mounts which allowed excessive engine movement when accelerating. Thereby, causing engine head-pipe to separate from exhaust manifold (causing a temporary exhaust leak) on hard acceleration. Apparently, the spring loaded exhaust bolts were allowing sufficient flex for the leak. And, thereby, creating the discrepancy in upstream and downstream O2 sensor readings.

See following thread: P0420 | PriusChat

Conclusion: most P0420 codes are related to worn out catalysts or bad O2 sensors (upstream, downstream or both). However, exceptions do exist.

*- oil catch cans have been known to prevent oil from getting to the catalyst and preventing the P0420 code from being repeatedly tripped.

**- However, 33-36mpg would be entirely understandable if you had one, or a combination, of the following issues: i.) low engine compression causing low power output; ii.) bad HV battery; iii.) driving very short trips where Prius fails to warm up fully; iv.) driving in extremely cold weather; v.) frequently driving on highway at speeds in excess of 75mph; or vi.) excessive oil consumption combined with low engine power.

 

Which codes (DTCs) were those, and what device or app are you using for retrieval?

None of the above will last forever, how many miles are currently on the parts in question?

 

This might be another thread resurrection I can follow up my 2007.
I have a 2007 Prius
The "Check Engine just lit up 2 nights ago as I was pulling in the driveway.
I put the scan tool today and its says "1 fault" and it displays this:
P0420 01
P0420 02
I was also a victim of a stolen catalytic converter and replaced mine with aftermarket 2 years ago. I bought the replacement aftermarket probably 120 dollars and have it installed professionally. Basically no leaks.
At first time replaced, it was running nicely with above 45 mpg on the screen. After a year it went down to 41-43 mpg.
My car is fully tuned up and serviced as what the book requires on maintenance service.
Presently it has 151k miles and eats a 1/2 a pint every 4500-5000 miles of driving. I use 87 regular gasoline.
What I did yesterday is I, I cleaned the throttle body and intake, run it half an hour, change oil and filter 10w/30 Quaker State synthetic (as I always use every 5,000 miles, air filter is clean and not due yet so I installed it back. I reset the check engine light using the scan tool and maintenance after the change oil. After 100 miles of city and highway driving, the Check Engine turn on again. I also want to add that besides the check engine light, the car still runs beautiful. Here is my last screen right when I was heading home. Any suggestions? Thanks.
If I need to change the 02 sensor and it could be a part of it, I wont mind changing them. with the original Toyota parts. The only thing that I cant afford to put back is the original catalytic. its too expensive.

 

A prius only has one catalytic. Which device/app are you currently using?

Would recommend techstream (dealer technician diagnostic software), or this thread :Gen2 OBD2 app review | PriusChat

Where in the NW are you, maybe another member with a hybrid compatible scanner can help you.

 

in Oregon. The other day the check engine light shut off by itself. Then it turn on again.

 

I know that @PriusCamper makes it down/threw the state of Oregon at times, and could likely lend a hand.

That scanner isn't going to be much help, you need something more robust, check this thread for some hardware/app ideas. Personally would get a cheap adapter and run Techstream, which isn't listed below.

Gen2 OBD2 app review | PriusChat

 

I have one in the laptop and I haven't used it. Like a techstream. Cant remember. I'll follow-up shortly.
The check engine light turns on, sometimes turns off for a few days..