potatomoonlight

So this is a problem that I've had for a while, though, yesterday I made an interesting discovery. A couple years ago I added some parts to the car: basically creating an E85 Cobb Stage 4 setup (but before the stage 4 kit was available). Everything worked well (and continues to be awesome). Except that, upon completion of the install, the rear 02 sensor stopped functioning properly, resulting in some O2 sensor code (I forget which code it was, but I don’t think it was P0137). Anyhow, I used the AccessPort to disable the code and disable the sensor. I ran like this for a while without issue until recently when Cobb removed the "disable rear 02 sensor" function. Now, I get the P0137 code. I figure I should just fix the issue. To that effect, I've tried replacing the rear O2 sensor twice. I've also tried all bung extension/reduction tricks. I've cleared the code 100 times but it always comes back after a couple miles. Then, yesterday, while I was doing some logging and map updates, I noticed that the stupid CEL wasn’t coming on. I even checked the "Rear O2 Voltage" monitor on the AP and it actually appeared to be spitting out plausible values. I parked the car for just over an hour, happy that the issue had spontaneously decided to resolve itself. However, my happiness was not to last…the CEL and P0137 code returned after a mile of easy driving. ****! So, what the hell is happening? Why did it decide to work yesterday? And how can I solve this?

Ok, so I have a theory: I think that the power to the heating element on the O2 sensor has failed. Considering that all the O2 sensors I've tried have thrown the same code, the problem would have to be upstream of the connector. Yesterday, when I was testing, I was pushing a lot of hot air down the tube…perhaps it was enough hot air to get the element warm enough to start registering voltage. It could explain why it worked yesterday and why it refuses to work when I just idle around town.

If the problem is, indeed, with the heating element not getting power, there are three possibilities:

1. There is a break in the wire somewhere. That would suck…I really don’t want to remove the interior to find a dent in a piece of wire.
2. The fuse for the thing blew. That wouldn’t be so bad except for the fact that I have no idea which fuse corresponds to the rear o2 sensor. I think this is unlikely as whatever else it shares the circuit with would be dead as well, and I haven't noticed anything else out of the ordinary.
3. My best guess is that when I disabled the O2 sensor with Cobb AccessTuner I actually disabled the heating function. Then, when they updated the program and removed that functionality, my vehicle was stuck with the sensor permanently off. Since the functionality is gone, I'm unable to turn it back on.

Please advise!

potatomoonlight

Quick update:

So it definitely isn't a fuse. It's definitely not related to an extended rear O2 bung (I had it sawed off and now the sensor tip sits basically flush with the wall of the exhaust pipe). Nor do I think it has anything to do with AccessTuner. I spent some time yesterday trying to identify exactly where and when the voltage is nonzero. And what I discovered is that the AP begins to occasionally report voltage (albeit only 0.02-0.04V) 5-10 minutes after I start the car. I haven't done enough testing to know whether that time fluctuates based on "how" I drive the car (i.e. does hard driving get the sensor functioning more quickly than idling). The voltage that I'm seeing does creep up over time and, given enough time, goes just above 1V (dropping to 0V when I let off the loud pedal). I have two questions for the community:

1) Does this sound like a sensor heater issue or an issue with the sensor itself? I think the circuit is find due to the fact that it eventually registers a voltage.
2) Someone who's running with a stock O2 sensor (and who's not throwing any codes), would you be willing to do a quick test and describe the behavior of your sensor? What's the min/max voltage you see at idle/cruise/wot? How long does it take to begin registering voltage after a cold/warm start?

Thank you! I greatly appreciate it!