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2012 Corolla Base L 1.8L Downstream O2 sensor only partially testing and preventing catalyst from testing


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Now, I know what you're thinking...bad downstream O2 sensor. We went through this last year with this car. Failed readiness for emissions. Exact same situation. I put in a new battery last year, reset and still the same after lots of driving. I had a local garage put on a new Denso O2 sensor that I found (it was the last one in our very large city at all of the auto parts store locations). Still the same thing after reset and a lot of driving. I looked up virtually every kind of drive cycle I could find, we drove it 600-700 miles on the highway and in the city during this fiasco, stop and go, warmed it up for minutes in the morning, turned on various things in the car while warming it up, let it run for a few minutes after driving. Everything.

Went to 2 Toyota dealerships. The first one wanted to change out the new downstream O2 sensor again. They finally did and it still kept doing it. Nobody knew why. They ended up eating the labor and part charge for changing the downstream O2 sensor because that wasn't it and they kept pushing to try it even though a new one had just been put on and 1/4th of the downstream O2 sensor tests had tested each time even after the second one was put on. They kept wanting to reset it and saying to drive it and it seemed like the "drive it" was their excuse every time at both dealerships. Second Toyota dealership, same thing. Reset, drive it. Having seen how fast it tests, it is within 20-50 miles of driving after a battery reset and everything tests when it is working properly.

At the second dealership, a Senior Tech looked more deeply at it. Last year, they said he traced the wiring somehow for the OBD2 from end to end. He told the service clerk to tell us that there were "a couple of broken pins" that he replaced and it passed emissions right there. However, this year as we're having the same issue, that Senior Tech has since had a baby and he has apparently retired.

So the service rep pulled the paperwork from last year and comments from that Senior Tech last year just say, "Test drive vehicle using drive pattern conformation above 45mph inc 2 30 second stops. Check monitors. Repeat drive cycle, all monitors complete and ready for emissions testing." However, it did not mention the two pins that were said to be broken last year, for some very odd reason.

How can I trace the OBD2 wiring to see if there's a short or something loose?

The battery is the same as the one newly put in last year before it tested fine with the Tech, so it is likely still good after just one year. It has had a new downstream Denso O2 sensor put on twice in the past year. It's safe to safe that's good.

I have a bluetooth device called BlueDriver that you plug into the OBD2 and use an app on the phone to link to it, do real-time Mode 6 readings and such. I can literally see when each thing tests, voltage/temp it is running at, etc. It also lets me grab full diagnostics as seen below.

When I reset it by taking the entire battery loose for 10-15 minutes, half of the readiness tests complete within 30 minutes or less of driving. Everything tests within the day except for: Only 1 or 2 of the 4 total Bank 1 Sensor 2 (downstream O2 sensor) tests test. This causes the catalyst to also not test because it apparently kicks in after the O2 sensor tests. It seems to vary between 1 test completed and 2 tests completed, then all 4 may go back to untested status or it may go back down from 2 of 4 tested to 1 of 4 tested.

What other things can I check? Where can I find a real wiring diagram that I can understand? Honestly, it seems like the Techs see so many variations of vehicles (even if made by 'their' company) that it's like they don't know what they're talking about. I'm tired of people saying "just drive it!" because that's obviously not the problem.

It's a 4-pin square plug going to the oxygen sensor. I unplugged that, checked the pins on it, looked at the pins on the female part of the plug and checked the female pins with the multimeter. A couple of those pins are pushing like 29.5 volts and one registered up into the mid-30's in volts. I saw nothing off about it.

I also took off both of the OBD2 plugs into the ECM/ECU under the hood and checked them all and I see no broken pins there, either. Last year when this same issue happened, I got under the driver's side dash and meticulously checked every fuse one by one and nothing was wrong. I checked all of the fuses in the box under the hood, as well. They were all fine. I'm puzzled.

Air filter and new iridium plugs (I gapped them and installed them myself) were put in around October 2021. There are no codes in the system. Anyone have any other ideas?

Tomorrow I'm going to try to trace each OBD2 connector from the ECU all the way back to the downstream O2 sensor and see if there are any connection issues. Though it seems to be getting a decent amount of power. I don't think the catalytic converter is bad. It was well-maintained meticulously with oil changes and all sorts of stuff before we bought it. I had already pulled the CarFax report and oddly enough, though we bought it about 40 miles away, they were getting it serviced at a garage about 2 miles from our house for all of those years.

My BlueDriver Smog Check Report10241024_1.jpg

My BlueDriver Mode 6 Report10241024_1.jpg

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My BlueDriver Mode 6 Report10241024_6.jpg

My BlueDriver Mode 6 Report10241024_7.jpg

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That's a tough one! 

you seem to be checking everything in the correct order and the only other items I would check is the integrity of the catalytic converter joints (the slightest air leak can cause issues with lambda readings)
An emissions gas check can highlight any air leaks by way of high oxygen content (should be zero at the end of the exhaust).

Otherwise, you could replace the oxygen sensor connections or even hardwire them to ensure no error with the connection terminals?

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What are your thoughts on this 4-pin plug connections? I've got these odd flat pins and then these connectors that look pretty wide. It's an aftermarket part technically, but Denso always seems to be what Toyota used and I put a Denso on myself before they put this new one on. It looks like the female pins are the ones on O2 sensor plug. I have been trying to figure out the best way to get both the top and bottom part of the pins to "close" together more, but yet also how to figure out how far I can do so and have them still line up with the straight pins on the male connectors. Any advice? I have the tiny flathead screwdriver, it's just really hard to get any of them to bend or move.

I'm also trying to figure out what kind of pins those female pins are. Are they C-shaped so that if the pins went all the way to the back, they'd still touch metal? Or just a female pin on the top and bottom where the male pin has to touch one of the top or bottom? .

The male pins appear to be okay, but the female pins look a little wonky/crooked. Also, now two of the four total tests have passed again after I messed with it.

As for tracing the wiring...about 6 inches after the male plug on the side of the car wiring, it goes into a bundle of maybe 20 wires wrapped together and from there it goes into some kind of odd cloth-wrapped larger bundle of bundles.



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I got my multimeter, unplugged the O2 sensor and tested the car side of the connector. I also used the "Live" feature on the OBD2 bluetooth device and as you can see, no voltage on part of the downstream O2 sensor (bank 1 sensor 2) with the car running and in park...however, note that the heater is apparently getting the data for the temperature of the downstream O2 sensor. As seen in the photos, I'm only getting voltage on the bottom two pins. On the top two, nothing at all. I suspect that this is why only 1 of 4 and 2 of of the downstream O2 sensor are testing.

I pressed the multimeter against the back of the plug to see if the upper pins were just bad on the male side with the pins, but there's no voltage there at all, either. It seems to not be getting voltage on the two top pins. On another forum, somebody told me that supposedly the O2 sensor "makes it's own voltage" since it is low voltage. Something still isn't right or it would be working, however.

Anyone know where I can find the "dummy oxygen sensors" that people used to use until they pulled them x years ago from the market? I can't seem to find one anywhere, even from private sellers. Would it temporarily work just to pass this year until I can figure out what is wrong? 

I still see it testing up to 2/4 and I think at one point even 3/4 of the downstream O2 sensors briefly. Now it's back to 1/4 presently and still the catalyst that won't test until this O2 sensor tests.

Anyone have any other ideas? At this point, I'm literally thinking of ordering a generic brand of O2 sensor for $30 bucks on Amazon and see if it is somehow a bad sensor working partially. Toyota just wants to "put it on the diagnostic machine" for $150 and that won't tell them much. Just a money pit taking it to them. I don't know what it is with their Techs in my area, but the smart ones have left and the ones that don't know what they're talking about just guess at things.





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