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Thread: O2 Sensor Issue / MIL-CEL with P0158

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    Icon922 O2 Sensor Issue / MIL-CEL with P0158

    Hi all-
    Armchair mechanic here, and having a devil of a time with an O2 sensor on my daugher's '04 WJ Laredo 4.0L 6-cyl. w/ 138,000 miles. The history is that we had some O2 sensor codes (don't recall all of them now), and poor mileage. Her boyfriend replaced all 4. Unfortunately, he didn't secure the wires on the 2/1 sensor (front, after mini-cat if I'm correct), and the insulation melted against the exhaust pipe, and it shorted, causing the fuse to blow (found out about the melted wire only after blowing a second fuse).

    On the 2/2 sensor, the replacement was a allegedly a OEM-type, but the alignment tabs on the male/female connectors didn't align, so they were filed down on the sensor-side connector. When all of this was done, and codes cleared (plus PCM reset via battery disconnect), I was getting a recurring P0158 code. Thinking I had a bad sensor, I replaced this one again and cleared the code. It would stay off for the duration of the initial trip (whether 1 mile, or 100 miles), then the MIL comes on and the code is set again (always the same P0158).

    Well the code says that sensor may be shorted to voltage, so I thought perhaps a the O2 sensor relay - so I swapped with the (same part number) horn relay, but no change. I should also say that I checked the wire-loom around the top/back of the engine by the firewall, and did find the loom starting to disintegrate, but it appeared all wires in-tact. I cleaned this up with some new loom, and put some silicon super-tape over the offending engine bolt.

    I'm now at somewhat of a loss, and could use some help. From here I'm thinking I could begin tracing wires or try yet another sensor (would go Mopar part this time vs. Bosch). I'm also wondering if this could be a legitimate code in the event of a bad mini-cat, injector, etc. (high-voltage indicating running very rich?). I guess last-resort would be to take it to the stealership and take my lumps, but need to do something soon as emission inspection is due.

    Thanks in advance for any help you guys can offer.
    2004 WJ 4.0l

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    I wonder if there might be a mixup as far as which sensor is which. This phrase indicates that possibility:
    didn't secure the wires on the 2/1 sensor (front, after mini-cat if I'm correct)
    However, the 2/1 sensor is before (engine side of) the mini-cat. The 2/2 sensor is after (muffler side of) the minicat. Then, just in case, I will also mention that the 2/1 and 2/2 sensors are both in the pipe that carries the exhaust away from the rear three cylinders. Was there any mistake identifying the sensors?

    You mentioned that you changed one of the O2 sensors that the boyfriend had already changed. Did your replacement have to have its connector filed down also or did it fit without modification?

    The cause of the P0158 code is pretty specific and the other possibilities that you mentioned would throw some other codes. Assuming that sensor identification is actually correct, I'm pretty sure that either the new sensor is faulty (replace with a Mopar sensor) or there is a wiring problem, like maybe the connector got buggered up.
    2001 WJ Limited, 4.7L, Quadra-Drive -- Kenne Bell Supercharger, Stillen Rotors with MM pads (F/R), Addcos (F/R), Kolak 3 inch exhaust with large case MagnaFlow, Rogue Chrome wheels, DRL activated, and Blaupunkt 3.5" dash speakers.

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    Thx 'Geezer - probably my bad in calling it 2/1, not 1/2. The one with the melted wiring was the one after the mini-cat, and for the front 3 cylinders. I had to replace it (didn't want to risk splicing and supposed signal degradation).

    If I read correctly, the code (P0158) should be for the 2/2 sensor, after the mini-cat for the rear-3. That's the one I changed (after the BF had changed it when doing all 4). I didn't have to alter the plug on the one I put in, but it is a Bosch (as are the other 3).

    What would the symptom/code be if I had a bad mini-cat? (with 138K miles, is that a concern?)

    Could a bad injector, plug, coil/wire, cylinder cause over-rich and high-voltage?

    This weekend I was going to try tracing wires, but might be willing to go with a Mopar sensor test, though my brain says unlikely after 2 fails with the Bosch on this one sensor, while the other 3 Bosch sensors seem fine.
    2004 WJ 4.0l

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    A bad cat (Bank 2) code would be P0432. However, that code requires both the 2/1 and 2/2 sensors to be working more or less correctly.

    The Bank 2 mixture is too rich the code would be P0175, but the 2/1 sensor would detect that condition as well as the 2/2 sensor.

    When working properly, the O2 sensor signals should be oscillating between about 0.2 and 0.72 volts with the average being close to 0.47 volts. There is almost no doubt that the P0158 means that the 2/2 sensor signal average if significantly above 0.47. The max possible signal voltage is 1.0 and the min possible is 0.0

    The O2 sensor has approximately 12 volts supplied to the sensor heater. Two of the sensor wires are used for this with one of them being a ground. Of the other two wires, one takes the sensor signal to the PCM and the other is a sensor signal ground. My guess is that somewhere in the wiring harness or in the connector there is a short between the signal wire and the 12 volt heater wire. You can check for this by unplugging the sensor wires and then measuring the voltages at the contacts in the wiring harness connector with respect to ground while the ignition key is in the ON position. The voltage at each ground contact should be zero, the positive heater contact should be about 12, and the voltage at signal contact should probably be near zero, but definitely not near 12.

    It seems to me that anything going on in the engine that could cause the P0158 (2/2) should also cause a P0152 (2/1), because the exhaust gases have to pass by both sensors.

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    That seems pretty straight-forward - thanks! I guess I was thinking that something in any one of the rear-3 cylinders would only show up on the 2/2 sensor, but makes sense that the 2/1 sensor would also see it - just pre-cat. And I haven't seen any cat-related codes, so that seems not to be the cause either. I guess I know what I'll be doing this weekend! I hope I wasn't to hasty with the wiring & loom repair - assuming all the wires were OK in there. Hopefully once I'm underneath I'll find the issue, otherwise I'll probably be checking the connector back to the PCM, and I'll need to find a wiring diagram for that. BTW - I have a ODB scanner that lets me monitor the O2 voltages in close-to real-time - it does show the 2/2 sensor pegged at 1.0v - no oscillation. The others seem normal.

    Thanks again 'geezer - I'll post an update afterwards - maybe/hopefully this'll be a help to someone else.
    2004 WJ 4.0l

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    Well I finally got time to dig into this. As it turns out, I didn't even have to take the multi-meter out of the box! Turns out the harness for the 2/2 O2 sensor was not connected tightly. Seems the "universal" Bosch sensor is not an exact fit for the WJ's connector, so pressing it extra hard closed the gap.

    So I thought this would take care of it once I cleared the code and put some miles on. Half-right! I then got P0135 and P0155 codes - heater circuit failures on both the 1/1 and 2/1 sensors. It was VERY cold (low teens) when this happened, and the MIL came on again. But after a few days of warmer temps, the MIL light went off (although the codes are still showing in the 3-click odometer display ?!?!).

    All 4 sensors are Bosch, so I bought a couple of NTK OEM's to replace the upper 2 if the MIL comes back. Maybe there's something to this "Jeeps-hate-Bosch" talk ...
    2004 WJ 4.0l

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    Hi, I'm having the same problem with my Bank 2 Sensor 2 and I've come to the conclusion from your information that the engine is sound because my Bank 2 sensor 1 isn't sending a code. I think I've narrowed my problem down to 1 of two things. Oil deposits on the exterior sleeve of the 02 harness, corrupting the exterior 02 reading. The O2 sensor must have a source of oxygen from outside of the exhaust stream for comparison. Current O2 sensors receive their fresh oxygen (outside air) supply through the O2 sensor case housing.

    OK so the reading I'm getting from that O2 sensor is reading between 0.999 and 1.025... this is too high.
    So I either have to clean the wiring harness and exterior of the Bank 2 sensor 2 and see if that works or replace the NTK 02 sensor with an NTK. Do you know of a cleaner that would remove engine oil off of the sensor well?

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