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My 2006 Toyota Prius is throwing code P0420 and I used a cheap OBD-II reader with Torque App on Android to check if my O2 sensors are working or not. I tried the maintenance mode to just run the engine but it failed to show any reading, so I drove the car on a freeway at constant 65 mph and took screenshots during the drive:

I put the other graphs just to make sure that the sensors would not be displayed in another bank. It appears that the sensor closer to the engine (O2 1x1) is showing nothing where it should have had a reading. When I stopped the car it briefly showed some readings but I could no catch them. The sensors after the catalytic converter looks like is varying very swiftly, which could indicate a bad catalytic converter.

Are those readings reliable enough to decide to replace the upstream sensor? I'm not sure if the lack of reading is because of a problem in the sensor or in the Torque App/Bluetooth reader.

Edit 1:

I took the car for another ride today and the O2 1x1 sensor provided some reading, although it was a constant reading, which is inconsistent with a working upstream O2 sensor (should vary right?). Interestingly the O2 1x4 gauge also provided a reading too, I didn't know I had one!

  • Do you know any auto parts store near San Jose, CA that does that? AutoZone website does not list OBD-II scanner in their tool loan program. – Gabriel Diego Feb 17 '16 at 19:15
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    Yes it is possible to have no signal from the O2 sensor. There is no default non-zero voltage for narrowband O2 sensors – Zaid Feb 17 '16 at 19:50
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    Also, was your car fully warmed up when you took these readings? – Zaid Feb 17 '16 at 19:51
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    Ok, i just wanted to make sure we weren't seeing cold start readouts. Something does seem to be wrong. Your RPMs shouldn't be negative. It does seem like your app is struggling to read from the OBD-II port. There might be an intermittent open somewhere which would explain why you were able to get readouts for a short period of time – Zaid Feb 17 '16 at 21:02
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    @DucatiKiller Honestly, not everywhere in the world you can get this service. AutoZone in USA offers this, but in most countries part stores don't. Code scanning is not free and done only by mechanics. – I have no idea what I'm doing Feb 18 '16 at 8:32
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The upstream o2 on a Prius is an AFR sensor stoich reading at idle is 3.33 volts. The application isn't able to read the correct PID. So of course it would read 0v all the time.

  • Yet it read by the end of the trip (I could not take a screenshot of that). That's why I was wondering if there could be a problem with the sensor (like a broken wire) or if that is a problem with the app. – Gabriel Diego Feb 18 '16 at 1:02
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    @gabrieldiego You took a drive and the app "briefly" read a value. Define briefly, a second? a minute? a millisecond? If you're not setting circuit high/low/no response codes this suggests that your AFR sensor is working fine. Which in turn suggests that the App isn't pointed at the right PID. Mode $6 on Toyota ECMs have an air fuel ratio sensor monitor as well as a Mode $1 PID. – Ben Feb 18 '16 at 1:33
  • It was for about 10 seconds that it showed some reading. I will drive the car tonight then I post if it reacts the same way. Zaid suggested that it could be that the engine was running cold, thus the reading only came when it heated. – Gabriel Diego Feb 18 '16 at 1:39
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    @gabrieldiego Do me a favor when you use the app again. Take a look at all the PIDs available to you there may be an AFR specific PID. Unless the AFR signal circuit is open there will never be a 0v reading on a Toyota AFR sensor. To confirm AFR sensor operation you can back probe the green (or red, I forget which) wire on the harness side of the plug. – Ben Feb 18 '16 at 1:44
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    @gabrieldiego From what I understand of the torque app it has predefined PIDs and the math necessary to convert the value you're given to something readable. Take a look at this video: youtube.com/watch?v=oWR6qsTQYHc You should also be able to add custom PIDs I'm unsure of how this is done but from what I gathered on the Torque forum you define a memory address and the math to go along with it. There should be an option in the app to do this. I'd suggest you take a look at the Torque forum: torque-bhp.com/forums – Ben Feb 18 '16 at 23:28

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