solution for P0420 and P0000 > new rear O2 sensor

My 2005 Prius with 125k miles has been showing codes P0420 and P0000 with increasing frequency for about the last 2 years. I had been able to clear them and pass emmissions inspection in the past, but that no longer works since the CEL came back on after about 20 miles and I now fail the test.

I read up on the codes and thought I might have to replace the cat, but I decided to first try replacing the O2 sensors before and after the cat. I could not figure out how to remove the connector for the forward cat without pulling the inverter so I put that replacement on hold and changed the rear O2 sensor in 45 minutes. I am happy to report no CEL after 150 miles so I think I solved the problem and hopefully will pass inspection this weekend.

I was too busy to do detailed component troubleshooting and decided to shotgun the problem with O2 sensors. I bought them at Rockauto:

DENSO​

2349056 (234-9056)
Oxygen Sensor

$ 108.79​

$ 0.00​

1​

$ 108.79​

DENSO​

2344623 (234-4623)
Oxygen Sensor

$ 57.79​

$ 0.00​

1​

$ 57.79​

I hope this helps others who come across this code in the future.

Good luck,

Doug