4 or 5 months ago I discovered the wiring harness that goes down to the upstream O2 sensor was laying against the exhaust manifold and this basically disintegrated the wire loom and protective sheathing around the 4 wires on the harness. The bare wires found themselves exposed and wrapped around each other causing a short which lead to my Jeep running like poop. My Auto Transmission would not shift correctly, Jeep felt like it was about to stall every time I stopped at a light or let up off the gas peddle. Eventually a CEL light came on (P0171). The OBDII reader said the jeep was running too lean.

Once I discovered the exposed wires touching each other, I first removed the old wire loom to inspect the wires going down to the O2 sensor for any other breaks or cracks. Then I wrapped each individually with electric tape as a temporary solution. I put them in a new wire loom and wrapped the wire loom in special heat reflective ducting tape. The above issues were resolved however it's been months and the CEL trouble code never went away and the transmission still kind of down shifts violently when I'm going 70mph and I let my foot slightly off the peddle but not completely off. I looked at my OBD2 on my phone using the app Torque and it shows the upstream O2 sensor is correctly bouncing between 0V and 1V. Driving, it'll usually bounce between .3V and .6V I believe. My Long Term Fuel trim is around -5 when idling and as low as -15 when on the highway, depending on if I have a lead foot. Short Term Fuel Trim bounces around from 0 to +5.

Seeing that the Long Term Fuel Trim is reaching such low levels, I figured something might be wonky with the wires or the O2 sensor still. Maybe the wires touching each other caused the O2 sensor to break. I tested the wiring harness connector for Voltage. It's only getting 4V on one of the black wires when the key is turned to the on position and the engine is off. I thought it was supposed to be 12v? I also put my probes into the O2 sensor's two heater element wires to check for resistance and I'm getting around 6.8 omhs which I think is correct. So my conclusion is the wires on the harness going down to the O2 Sensor probably need to be cut and new wires spliced in. Does this sound like the problem?

My Questions:
1) Is the wiring harness supposed to have 1 black wire with 12v feeding to the O2 Sensor's heating element when the Jeep is on and engine is off?
2) What are the correct ohm readings when testing the O2 sensor's two black wires supposed to be?
3) What gauge wire is used for the wiring harness down to the O2? 18?
4) Does anything stand out from my post that cause be causing the P0171 code?

My Jeep:
99 Jeep Cherokee XJ

 

I would download the FSM from Cruiser54 webpage if you dont already, and check that for specific data on O2 readings

The O2 sensor does switch "on" and "off" I seem to recall around 0.8 and near zero volts, others may know more if they have live OBD data running

otherwise

There are many different solutions for the P0171 code because there can be so many different causes.
Here are the most common fixes to the P0171 code.
Most common from the top.

• Replace faulty vacuum hoses or gaskets around the intake
• Repair other intake leaks
• Replace PCV Valve
• Replace the fuel pump/fuel filter/fuel pressure regulator or repair wirings
• Replace EVAP valve
• Replace O2 sensor(s)
• Replace EGR valve
• Replace MAP/MAF sensor
• Repair exhaust leak
• Replace coolant temperature sensor
• Repair faulty wirings
• Replace ECM/PCM (rare)

If it was me, I think I would start with ensuring harness/plug integrity, then maybe a new O2 sensor (or used OEM sensor & harness from a wreck)
Someone else on here shorted this harness, and afaic the outcome was a blown PCM, maybe check the archives
Good luck

 

Thanks, the FSM is what I've been using to try and figure this out. The CEL Code of P0171 came on when the exposed wires in the harness going down to the Upstream O2 sensor were touching for a few days, causing a short. So I'm 99% sure it's not a Vacuum leak or an exhaust leak or dirty injectors. I have gone through and checked most of the vacuum lines ad everything visually looks fine.

The FSM says the resistance on the O2 Sensor heater element wires "should be between 4.5 6.5 ohms and 7 ohms" between the two heater element wires.
Also says that the O2 sensor should oscillate between 0v and 1v
Both of which seems to be the correct readings for my Upstream O2 sensor. I get 6.8 ohms and the O2 sensor oscillates fine, never flat lining.

I'm having trouble finding information about the wiring harness to the Upstream O2 Sensor. Specifically how many volts it's supposed to feed the heating element. One of my wires (Black with Green Line) is getting 4V when the key is turned to the on position with engine off. All I could find was this:
A 12 volt signal at this input indicates to the PCM
that the ASD has been activated. The ASD relay is
located in the Power Distribution Center (PDC). The
PDC is located in the engine compartment (Fig. 2).
Refer to label on PDC cover for relay location. The
relay is used to connect the oxygen sensor heater elements,
ignition.

So I'm guessing I am supposed to get 12v on one of the wires?

 

In order to be systematic, after examining the circuit diagram, I would unplug the harness or snip the 12V wire, and see what readings you get.
If an expected 12V is coming down the line, I would replace the O2 sensor if not already tried

You may have more than one fault.

please report your findings

 

I found out from another forum that the 12v only shows up for a few seconds if the Jeep engine is not running. This is why I wasn't seeing it when I turned the key to the on position. I needed to have the multi meter right next to me in order to see it. It turned out to be a the green wire with white line that had the 11.5 / 12V. I'm also getting 4v on the black with dark green striped O2 signal wire when the jeep key is in the on position. I haven't had a chance to check the Manual yet to see if that's normal.

I've since unplugged the battery to reset the code and for the past 2 or so days nothing has come back on (knock on wood). The sensor shows up on my OBD2 reader showing that it's correctly bouncing between .1V and 1V and never flat lining. When Unplugging it to check resistance again on the heater element it's showing 6.8 ohms (I cold tested it). My Long Term and Short Term Trims are still lean though. At idle They're around -2 or -3. On highway driving or going up hills I'm seeing -6 and short spikes to -18 when I floor it to get up to speed. So I'm thinking the O2 sensor itself is fine. I'm going to continue to monitor it until the check engine light comes back on.