96 xj, PO138, wiring issue, 5-90 and others, thoughts?

Well, recently my XJ has been throwing a po138 code. A few times I would just keep deleting the code as it only came about every few months or so, but recently the light is coming back on much more frequently. I have no associated drivability issues (as I would expect from a rear O2 sensor code).

After doing some reading on here, I'm more concerned with a wiring issue than an O2 sensor itself, and I'm not going to just throw a 60$+ sensor at it. I wish I had a 96 FSM though

Anyways, here is what testing I have done so far. I took the harness apart at the sensor, and ID'd my wires:

Green/orange=heater 12v+
Black/white=heater ground
Black/blue=sensor signal
orange/white=sensor ground

I did the same tests at this location AND in the engine bay at the rear O2 harness connector there, thereby eliminating the actual harness segment underneath the jeep as the test results were the same at both places.

I checked for voltage at the GN/OR heater 12V+ wire (with key in the on position, engine off). 0 volts. I also double checked this against a known good ground, still 0 volts. Also no voltage at the BK/WH or BK/BL wires. I do however have approx 4.3 volts at the OR/WH wire (sensor ground).

I am somewhat confused by these findings... I expected 12v at the heater supply wire (green/ orange stripe). I havent had any heater circuit codes. I checked for power at fuse #20 in the PDC, which is a 30 amp (auto shutdown relay), and I have over 12 volts with the key on or off, and the fuse is good. Do I need to be engine running in order to see 12 volts here? I was under the impression that key on, engine off would do it.

And the second thing here... I was hoping to see the heater supply wire 12v had spread some voltage to the black/blue sensor signal wire, hence resulting in the PO138 (sensor volts too high). Apparently not the case. So why am I seeing 4.3 volts in the sensor ground wire? Where is that voltage coming from if I'm not seeing any in the 12V heater supply wire.

I may be leaving some things out, so ask away.

Have you had your computer re-flashed? '96s were recalled for this, and it may help your computer recognize the new hardware. I hope you either bought a Bosch O2 sensor, or got one from the dealer. I've had some bad luck with aftermarket sensors. 1996 was the first year of OBDII, and maybe you have an early model? Or, a very late one, which is a lot of '97 under the hood? You may end up at the dealer to get this sorted out, like I did with my long-gone '98 ZJ, not recognizing the new O2 sensor.

No, the jeep has never gone in for the recall regarding the cat or reflash. Original o2 sensor, cat, etc.

Update. Ok, so with the engine running, I am getting 14.0 volts at the GN/OR heater source wire 12v+. I guess it has to be running to get that voltage there.

I still have 4.26-4.3 volts in the OR/WH sensor ground wire. Again, this is at the plug in the engine bay, so the problem exists somewhere in the engine bay. I get 4.3 volts wether I go to an engine bay ground (with the black test lead) or if I go to the BK/BL sensor signal wire. SO if the 4.3 volts is in that circuit, it seems logical that is what is causing the PO138, as IIRC that signal should never be above 999mV or it throws the code. Now I need to now how 4.3 volts is getting into the OR/WH sensor ground wire, and where. Where exactly does this wire lead to in the engine bay... it goes from the plug up into the main harness along the back side of the bay. This leads either to the PDC or the PCM.

I may take it in at this point for the recall and see what they say regarding if this is related. I still dont know what happens inside the O2 sensor though, and is the 4.3 volts that exists in the ground wire what the sensor converts into the return signal for the PCM to see. hmm...