Downstream o2 sensor reading 0.0v sometimes?

[Mac]

I was having a P0420 (Catalyst Inefficiency code) on my 2001 Toyota Sienna, so I plugged in my new BlueDriver scanner (great little gadget!) and found this:

1. Downstream o2 fluctuates slowly, from about 0.7v to occassionally flatlining at 0.0v.
2. Long Term Fuel Trims at around 10-15%
3. Both upstream o2 sensors stay around 3-4v consistently, with virtually identical readings at any given time.

The data readouts can be seen here: BlueDriver o2 sensor readings

So, I replaced the downstream o2 sensor, and the problem still reoccurs with the new one. The old one was old dirty, with black splotches, the new one was shiny, and I think I installed it properly, but they both seem to be behaving the same. What should I look into next?

Note: I also have a very slight fluctuating idle (mostly when the vehicle first starts), and a knock sensor code. I’m broke enough I can’t take this into a shop to diagnose, or replace the knock sensor/harness/assorted gaskets yet, though, but I include that info it might help a diagnosis. I’m preparing to move cross-country and I’m trying to determine whether or not to hold off until the issues are fixed. Any help would be greatly appreciated!

Topic

[college man]

With a p0420 the down stream o2 is reading that the cat is not cleaning as it should be.
Your cat is worn out. Nothing wrong with the downstream o2.

[James]

[Mac]

Thanks much guys, I figured the cat might be the problem, although I used an IR thermometer on the cat after driving for an hour, and it showed the input at like 180 degrees and the output at like, 230 or something (I forget the exact numbers, but the output was higher number by 40-50 degrees). I read somewhere that was typical for a three way cat because they were more efficient or something. It’s hard to find good specific data on what the temperature readings should be.

1. On this vehicle it’s a three-way warm up cat system. Is there likely to be a single weak link, or is it necessary/recommended to replace all 3 parts at once?

2. Might it damage the engine to drive it for a 3,500 mile cross country trip, or is the primary issue fuel economy?

3. The Knock sensor on bank 2 needs replaced as well, (which I believe means the ignition timing is delayed), and also effects fuel economy. Which issue should take priority?

[James]

I personally would address the knock sensor 1st. The bad cat situation affects the enviorment and techncially should be addressed but the knock sensor is more in tume with pcm detection of engine vibration/ noise. Bad knock sensor can also put out higher emssions while probably not could be affecting cat

[Author]

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