Still trying to diagnose the cause of P0420.

It looks to me that the catalytic converter is the problem since you can see the A/F ratio sensor voltage swinging back and forth, as it should. The post-cat oxygen sensor voltage should be more static - but since it also is swinging back and forth, that points to a failed cc.

 

Personally, if a new catalytic converter is under $200, I'd just get it fixed. With all the money you've saved on gas you've more than enough paid for a new cat.

I dont know where you live, but here in Michigan you can buy used catalytic converters from a salvage yard. Find a low mileage one for $75 and install that.

Or the least preferred method, you can just ignore it all together or find a P0420 "cheat". Ive seen them on the web where a resistor is placed in the rear O2 sensor plug that tricks the PCM into thinking the catalyst is working fine. Either way, some people get the P0420 and never get it fixed. (still I would - I hate a CEL on the dash and also hate rigging things). To me, 150,000 on a Prius is not yet to the point of not investing in repairs but its your car and you know it best. Maybe with your finances, a used cat would be an option?

 

3 words for you: car-part.com

Its a great site that will search the salvage yard database wherever you set it to search.
For whatever reason, here in Michigan salvage Prius are EVERYWHERE. Our craigslist here in Metro Detroit is always chock full of salvage Prius being sold for parts, or just plain parts being sold individual.

 

I have the same issue. I looked around, and found that the CAT is probably dirty and cleaning it will rejuvinate it at least for a while. There are several commercial products, such as CATAClean. Some UTUBE video suggests putting a gallon of laquer thinner in the gas, which I am afraid may damage the bladder, or removing the CAT and soaking it in soap and water. If you were to look in the CAT, you would see that the honeycombs become dirty and clogged. The cleaning seems to help this. Other sites claim that the PO420 occurs way before the CAT is bad enough for the emissions to be out of compliance. One "cheat" method moves the O2 sensor back out of the exhaust flow, slowing the response, and therefore reducing the variation in the output signal when the self diagnostic for this test is run. This could work, but is a visible exhaust system mod. Another method is to replace the sensor with a resistor, which will provide a fixed voltage. If the voltage matches the nominal CAT voltage, this would work, since no variation in output could occur. Another approach that I came up with is that a large capacitor could be added across the sensor to slow its repsonse electrically, providing a similar result as moving the sensor out of the flow, but much easier to install putting it in the passenger compartment next to the computer. This should, unlike the resistor fix still allow the CEL to work if it becomes way out of spec.