EDIT #2: I rescanned, looking at the correct data, and found the rear O2 sensor was definitely bouncing up and down with the forward sensor's lambda reading. Since I have looked for leaks and couldn't find any, I decided to just go for a new catalytic converter. I have read that you can use propane to create a rich condition and a vacuum leak to force a lean condition, and this is a more thorough test to make sure your sensors are working properly, but the rear O2 sensor moving with the forward, and not having any leaks, I feel confident enough that this really is a bad cat. I'm going to replace it myself, and I'll make sure to post the results. I do wish I knew more about why the cat went bad, as I understand there may be an underlying problem, and even if a new cat fixes the code, it may go bad quickly due to the underlying issue. But then, the car has 170000 miles, so it's surpassed its life expectancy.
EDIT: the red/blue plots are Fuel/Air Commanded Equivalence Ratio. I think I need to put this on hold and get more info, but I don't know how...
My 2005 Prius is throwing a P0420 code (catalytic converter). I just got a scan tool that works on my phone, and got the two plots below.
In the first picture, I was coasting (which shuts the engine off) then I hit the throttle, momentarily, then coasted again. I'm noticing that when the engine goes off and starts up again (green upper-left RPM plot), the o2 sensor readings (big plot with red upstream and blue downstream) go nuts. Is this normal?
In the second picture, I took my foot off the throttle, let the RPMs drop (but the engine didn't stop) and then hit the throttle. You can see that under normal conditions the o2 sensor is fine (red wobbles up and down, blue is mostly constant), but then I hit the throttle (RPM's pick way up) and the readings of the o2 sensors move quite drastically.
The catalytic converter code was cleared and the test passed (an Evap test is still pending) but I've cleared the code many times, no luck. I have tried the magic bottle of cat cleaner, followed the instructions closely, but it didn't help. Now that I can read codes and plot sensors, I'm hoping to diagnose the problem instead of just throwing stuff at it.
Also, everything I read says o2 sensors operate around .7 volt, but mine seem to center around 1V. Is my car just different? Could it be the Dash Command software not interpreting the data properly?
I can add any additional test conditions anyone asks for in the comments, and would like to say thanks in advance, I know how smart and talented this community is.