Is this the right way to diagnose P0420?

Hello fellow Gen 2'ers, I've recently got the P0420 error code for "Catalyst System Efficiency Below Threshold (Bank 1)". RIP.

After some reading on these forums, I'm trying to figure out whether it's the upstream A/F sensor or the downstream O2 sensor, or worst of all the catalytic converter itself that's at fault. But I'm getting some weird readings from the sensors that I wanted to run by y'all.

Here are some data plotted vs time. The first image ("there") is about 15 minutes where I was occasionally going on and off the gas pedal on the highway to see what each sensor does. The second image ("back") is another 15 minutes with a more engaged gas pedal and loaded engine.



The upstream sensor is at 3.95-4V at steady state. Shouldn't it be 3.3V? I'm plotting these PIDs with the Torque app:

• [PRIUS]Engine Speed(RPM),
• O2 Bank 1 Sensor 1 Wide Range Voltage(V),
• O2 Bank 1 Sensor 2 Voltage(V)

Any idea if this is one of the O2 sensors or the cat itself that has an issue?

If someone wants to see more data, I also logged the following:

• [PRIUS]Accelerator Pedal Angle(%),
• [PRIUS]Combined Power (hp)(hp),
• Air Fuel Ratio(Commanded)1),
• Air Fuel Ratio(Measured)1),
• Catalyst Temperature (Bank 1 Sensor 1)(°F),
• Catalyst Temperature (Bank 1 Sensor 2)(°F),
• Fuel Trim Bank 1 Long Term(%),
• Fuel Trim Bank 1 Short Term(%),
• Mass Air Flow Rate(g/s),
• O2 Bank 1 Sensor 1 Wide Range Equivalence Ratio(λ),

 

Ok, so for whatever reason the images weren't posted at full resolution

Here are the interactive versions of the two screenshots, and you're able to drag-and-zoom the x-axis at its corners (when the mouse pointer changes to a left or right arrow) to the appropriate times and see some more granular data:

• 16:36 to 16:53 here: https://jenesuispasbavard.github.io/rpm-vs-o2both-there.html
  
• 17:28 to 17:43 here: https://jenesuispasbavard.github.io/rpm-vs-o2both-back.html

 

I mean I'd rather spend $50 than $200 incl. welding if the problem is with a sensor rather than the cat - unlikely I know. And plotting graphs is fun (to me)!

In the following pictures, "there" route was variable engine load, "back" route was more constant load.

Looks like Long term fuel trim stays near 9.375% for most of the time, and both short- and long-term fuel trims stay mostly within ±10%.



Measured ratio does match up with commanded ratio when engine is under load, but not when I let off the gas (not sure if that's normal).



Interactive plots here:

• https://jenesuispasbavard.github.io/rpm-vs-trim-both-there.html
  
• https://jenesuispasbavard.github.io/rpm-vs-trim-both-back.html
  
• https://jenesuispasbavard.github.io/rpm-vs-af-both-there.html
  
• https://jenesuispasbavard.github.io/rpm-vs-af-both-back.html

Guess it's the cat, RIP.

 

Good to know there's no welding involved! How long do these aftermarket cats last compared to OEM? Judging by the service history I think this is the original cat in this 203,500-mile Prius (I bought it at 142k). I don't expect the new cat to last another 200k.

I guess I should wait a while to see if my engine is burning any oil too (so far I had a spell of 1qt/3000 miles but that seems to have decreased after a round of BG EPR/MOA/44K). State emissions inspection isn't due for another three months.