My 2004 Toyota Camry with 170k miles is pulling a P0420 code, which I've read can be due to a bad catalytic converter or O2 sensors. I have an OBD-II tool with the DashCommand app on my iPhone. Looking at the logs, O2S12.O2SV (downstream) seems to give reasonable readings (between 0.5 and 0.9V). However, O2S11.O2SV (upstream) is giving a a flat line of 0V always. But, there is a WO2S11.O2SV which is giving readings between 3.5V and 5V.

Is the O2S11 PID not used for these Camrys, but instead the WO2S11? Or are they completely different sensors, and my upstream O2 sensor is dead?

Any web pages with more information are welcome also. Thanks.

  • Could you please share a follow-up of your issue? I am in a similar situation at the moment. Thanks!
    – MaSlo
    Commented Dec 30, 2017 at 23:47

1 Answer 1


According to research I've done on the web, the W in WO2S11 stands for wideband, meaning it can calculate much better than a standard O2 sensor. This, I would assume, is what your car is utilizing to get its O2 reading and not the O2S11. And yes, that would be for the bank 1 sensor 1 (upstream or before cat).

Your issue is not with the B1S1, though, it's with the B1S2 side of things, or the catalytic converter. The P0420 is specific to the efficiency of the cat. This can mean one of two things. Either the O2 sensor is bad or the cat is bad. Considering the cost of between the O2 and the cat, I would highly suggest you do something with it prior to changing out the cat. In your question, you state that the B1S2 sensor is giving reasonable readings between .5-.9v. This could be okay, but depending on what it is actually doing will dictate whether the cat is showing as good or bad. When looking at your live data, you want to see that the front O2 sensor is fluctuating wildly. The rear one, however, should stay fairly steady. This video by Eric The Car Guy is decent example. He is a little bit long winded, but he does explain things fairly well. You'll see what I'm talking about with regards to checking the cat by two different means, one being live data the other being by temperature. Basically to check the temperature of the cat, once you have ran your vehicle up to operating temperature (very important), you'll check the temperature of the cat right where the exhaust dumps into it with a laser thermometer. If your cat is good, it should read hotter at the rear of the cat than it does at the front. You want to ensure you are checking as close to the cat inlet/outlet as you can. There should be a stark difference between the front and rear. (NOTE: Unlike what Eric The Car Guy says, catalytic converters work on three gasses: carbon monoxide (CO), non-burnt hydrocarbons (HC), and nitrogen oxides (NOx) ... he's just mainly talking about NOx in the video.)

If it isn't plainly clear that the cat is bad by the above tests, I would highly suggest you try something else before you replace the cat, and that would be a Seafoam Treatment and with a detailed write-up on ls1.com. This process will cost you an hour or so of your time and a couple cans of Seafoam. This process is much cheaper (as long as it solves the issue) than replacing the O2 or the cat. Please note this may only solve your issue for a time.

Your next step in this endeavor is to change the O2 sensor. While this is probably going to run you (if done yourself) another $50-60 (at least here in the States), but again, much less than a cat is going to cost you.

If all else fails, you'll need to replace the cat. This is a much more expensive option, but it should fix the issue.

  • Thanks, its then most likely my cat, as the downstream O2 sensor follows the upstream O2 sensor fluctuations (e.g. exhaust is passing through without being cleaned). If I can get my hands on an IR thermometer, I'll double check with the temperature measurement. As for Seafoam, I did something similar with carburetor cleaner on my throttle body. It works for awhile, but eventually the CEL comes back.
    – Michael
    Commented Mar 9, 2015 at 20:03
  • @Michael ... Yah, the Seafoam is a cheap "try it first" ... sometimes it works, sometimes it doesn't. It's a lot cheaper than a cat, that's for sure :D Also, carb cleaner won't do the same thing as Seafoam or others like it (Royal Purple cleaner). Commented Mar 9, 2015 at 23:56
  • @Pᴀᴜʟsᴛᴇʀ2: As for the rear o2 sensor signal, service manual for 2002 Camry (note the different year, though probably not relevant) contains a section "Emission Control System - on-vehicle inspection". At stable 2500 rpm, the rear sensor should fluctuate, see the screenshot. Being in the same situation as OP (getting P0420, having similar sensor readins), I am unsure what to think about my cat...
    – MaSlo
    Commented Dec 30, 2017 at 23:57

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