Issue with Post Cat O2 Sensor

I have had a check engine light come on earlier this year: So I went and bought two new Bosch post cat sensors and installed them a few months ago. Then the check engine light came on again a few weeks ago and once again, it's for O2 sensors: P1276 is for the PRE CAT sensor, bank 2, it's old. I did replace both pre cat O2 sensors in 2017 with 45K miles, the car now has 62K miles. They were Bosch OEM sensors. So I started digging into it. When I hook up the Durametric, the post cat sensor on bank 2 basically has no output: Here is my fuel trim at idle: And on throttle fuel trim, you can see bank 2 is further away from 1, I will have to research this again, can't remember exactly what it means, will watch the Tony Calis video again. Then I broke out the T10 scanner and it's much easier to see. In this one, I was idling until about 1/3 through the graph, then I took off and drove. Kept driving and here is the continuation. What I am seeing/observing: Checked the voltages at the plugs to the O2 sensors in the engine compartment, 0.45 VDC on the signal wire for all sensors with the ignition on, engine off. 12 VDC for the heater circuits at all connectors. Unplugged the post cat bank 2 sensor and verified that I had the code for no communication/open circuit. Plugged everything back in and then did the test drive. Bank 2 post cat signal is down in the dirt. I didn't even think it was doing anything (DOA) until I saw that it did have the small blips under load. So I can say the circuit is most likely working correctly. Maybe a bad connection at the DME or a bad O2 sensor. Bank 2 PRE cat signal does not seem to have the highs and low as compared to bank 1, like it is lazy. Maybe that is why I am getting the P0159 code. My thinking would be to buy a new bank 2 post cat O2 sensor, install it and see what the results are. I could swap it with bank 1, but I remember one of them being a PITA to get out, would rather just spend the money on a new one. I will have to check what the warranty is as I bought them from Rock Auto. If it works, I think I would then buy the pre cat O2 sensor for bank 2. But as I write this, maybe I should just get both on that side. Thoughts?

Well, I looked it up on rockauto, the O2 sensors have a one year warranty. It appears easy to make a warranty claim there, so I bought a new post and pre cat sensor for bank 2. $153 shipped and I should get about half of that back if the new one works and I send the old one back.

Keeping an eye on this story and hoping it was some issue with the replacement O2 sensor(s). Simple simple�ish

I had this happen with OBD codes for oxygen sensors on a different vehicle. They were replaced and the code persisted. It was the ECM that was faulty. It was rebuilt and has been fine. It did also throw another code for the MAF at the same time and this led us to suspect the ECM or DME in this case. The oxygen sensors were also interesting to remove . You have a very thorough troubleshooting process, more than I was.

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I hope it's not the DME, but very well could be. Bad solder joint would be my guess if it is.

It may be the pins in the dme plug. I had to repin all the o2 sensor wires in my harness because a tech used too large of a probe. They r .3mm pins and they can be spred. They should be back pinned with a spoon if u want to check at the dme. Also double check the wires all the way back to the dme by checking for continueity. The rodents really like o2 sensor wires for some reason.

So I installed the new post-cat O2 sensor on bank 2 and IT WORKS! When I did pull it out, I could see some very small white whisker sticking out of the probe end with the hole. I touched it and it basically disintegrated or dissolved. It was smaller than a human hair. I don't know if it had anything to do with it or not. To recap, here is BEFORE: I installed the new post cat O2 sensor and this is AFTER: You can see it is no longer DOA. I have not yet changed the pre-cat sensor on bank 2, you can see it is slow to react, I wanted to do one at a time. Will post when I pop that one in and log it. I returned my DOA O2 sensor to Rockauto (just a few clicks on their website, printed a return label for $6) and not only did I receive my initial purchase price back, but original shipping cost! Thanks Rockauto!

@Paul Waterloo thanks for posting an update! I'm having a somewhat similar issue with the heating element on post cat O2 sensors but need to further investigate to ensure wiring is good. Had a rodent in there but didn't see much damage and wiring looked ok so far. My sensors are relatively new, about 2 years old and worked fine in my old cats. I had transferred them to an x-pipe and that's when the issue began. I did have to heat up the flange to get them out....I was careful to heat up the flange and not the sensor itself but wondering if that heat could have damaged the heating element in it?

I would not be surprised if it did damage it. If you unplug the O2 sensor in the engine compartment, turn the ignition on and measure the voltage between the two silver plated connectors/two white wires (those are on the O2 sensor side, but figure out which ones they plug into on the engine side) you should have battery voltage, 12 VDC. If you have 12 VDC, the power supply is good. The grey wire is the sensor wire, it should have a voltage of of about 0.45 VDC, but that does not sound like it has anything to do with your issue, but just something to check. Then measure resistance across the heater wires on the O2 side, you should get approximately 3 ohms, here is my pre cat sensor that I just measured. The post cat sensors are the ones with the grey connectors, the pre cat are the black connectors. You can always unplug one of those and measure the resistance of that also so you can easily compare with your meter.

Thanks for the testing tip! I have an analog voltmeter so will have to give it a go soon. I purchased mine from Pelican and they qualify for the replacement, which basically means I may need to buy them again and will end up with credit on the account.