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suburman 04-09-2014 11:11 AM

O2 sensor problems. Need a master mechanic!!
Greetings fellow Jeepers. My 2005 WK, 3.7, 4WD keeps getting a P0161 DTC for the Bank 2, sensor 2, O2 sensor htr circuit. I found out that the connector was melted and grounded against the CAT. I corrected the melted connector and still get the same DTC. I checked the ground and power wires at the sensor connector (car side) and got 0 volts (brown ground wire) which connects to one of the white wires on the sensor and less than a volt on the power (green/black) wire that connects to the other white sensor wire. This was with Key on, Engine off. Got the same results with engine running. I probed the black/green wire at the huge (30+) wire connector coming out of either the PCM or the adjacent fuse/relay box (not exactly sure which) and got virtually zero volts to the bad sensor. I probed the corresponding black green wire (same big connector) going to Bank 1, Sensor 2 and got 13.46 volts. The bad sensor is a new NTK and the resistance across the white (heater coil) wires is per spec (4.0 ohms) so I'm confident the sensor is good. I also swapped some relays in the fuse box with same results. What else should I check before I give up an declare the PCM bad? I've checked everything I know to check. Any ideas are appreciated. Thanks, Scott

ALPINEST4RS 04-09-2014 11:29 AM

Re: O2 sensor problems. Need a master mechanic!!
1 Attachment(s)
Make sure its pinned right.

It sounds like the connector is pinned wrong. The Brown wire is not a ground wire, its a sig wire.

suburman 05-09-2014 10:30 AM

Re: O2 sensor problems. Need a master mechanic!!
Thanks for the reply Alpinest4rs. It was actually the brn/gy wire. I couldn't see the gray stripe in the dark. Here's some more diagnostics I've done since then.
'05 WK, 3.7L. DTC P0161 – O2 sensor htr circuit malfunction. Connector at the sensor burnt up and shorted against the CAT. I replaced the connector and O2 sensor but still get a P0161 code. I swapped the two downstream CAT sensors and the code didn’t follow the sensor (still P0161) so I know the problem is in the circuit. I also swapped the relay with the same AC relay and still got P0161. I ran the tests in the Factory Manual and got the following results. With KOEO the voltage on the brn/gy wire on the sensor harness at sensor connector (with connector disconnected) was .72V and shows continuity. When I start the car, the voltage goes to 11.17V and then drops to approximately 7V-8V after a few seconds. Resistance between the C-3 connector at the PCM (pin #10, brn/gy wire) and the #2 pin at the sensor connector (same brn/gy wire) was 2.1 ohms. Resistance between pin #1 at sensor harness and ground is 9.8 to 11 ohms. This resistance should be less than .5 ohms so it indicates a problem. Is there anything I can by-pass or jumper to see if this drops to .5 ohms. The bk/grn ground wire from the sensor heater connector routes around the back of the firewall and is impossible to access to check for a short. Any help or advice is appreciated. Thanks, Scott

jeepgcoman 05-29-2014 09:18 PM

Re: O2 sensor problems. Need a master mechanic!!
If the O2 sensor wiring shorted out against the cat., you may have toasted that part of the circuit in the PCM.

suburman 06-08-2014 08:46 PM

Re: O2 sensor problems. Need a master mechanic!!
Thanks for the help jeepcgoman. the PCM was my first thought also but when I check the voltage at the sensor end of the brown/gray power wire coming from the PCM I get 12 volts at first and then after about 3 seconds it drops to about 8 volts and holds steady. That's with the engine running. The brown /gray is supposed to be the power to the heater element from the PCM. Would the PCM be defective if it's putting out voltage to the non-ground side of the heater circuit? I suspect there may be a problem with the black/green ground circuit that comes off the ground side of the heater circuit because the resistance between the connector at the sensor and the connector at the junction box plug shows a pretty high resistance reading. The manual shows two splices on the way to the connector for that ground circuit. Is there a way to by- pass the splices and create a direct ground to test and see if that's the problem? Thanks for the help. Scott

Frango100 06-08-2014 10:33 PM

Re: O2 sensor problems. Need a master mechanic!!
3 Attachment(s)
Seems you have a service manual, but if not, then here three pages from the manual. There is only one splice in the ground wire for both rear O2 sensors, S146. Furthermore in the line is a connector, C104, but that one is common for both rear sensors and since the other rear sensor is ok, the connector C104 should be ok as well. There could be a problem with splice S146. Which junction box plug are you talking about where you measured the resistance from?
If you can not get a test wire directly on the O2 heater ground pin 1, then there is no other way than to remove some of the isolation of the wire and temporarely fix an other wire to it and put it to ground. If the message can then be errased, then you know that the problem is in the ground circuit and you could run a parallel wire to a known good ground point.

tomk 06-09-2014 04:48 AM

Re: O2 sensor problems. Need a master mechanic!!
1 Attachment(s)
AFAIK the O2 heater ground doesn't go to the PCM or the Junction Block. It goes thru C104 (an inline connector) to ground.

Frango100 06-09-2014 09:42 PM

Re: O2 sensor problems. Need a master mechanic!!

Originally Posted by tomk (Post 1034447)
AFAIK the O2 heater ground doesn't go to the PCM or the Junction Block. It goes thru C104 (an inline connector) to ground.

True, ground doesn´t go via the PCM, but the power does. C104 is used for both rear O2 sensors, so should be ok (otherwise the other rear sensor should give a fault as well). So what is left over is the wire from the sensor to the splice S146 or the splice itself.

suburman 06-12-2014 10:02 PM

Re: O2 sensor problems. Need a master mechanic!!
Frank and Tomk. Thank you both for the quick reply. Thanks for the pics Frank. I have the same manual and had actually already printed two of the ones you sent. I'm getting an 038 code now which is the heater circuit on the other rear sensor and they both go through the S146 splice to ground so I'm thinking that may be the problem.
Tom, I see in the diagram that the bk/lg wire is the ground that goes to C104 and then to ground. The Ground Distribution diagram on page 8W-15-5 of the manual shows that a bk/lg wire comes from every sensor to ground via S146. I thought I read somewhere that the auto shutdown relay was in the heater circuit but that doesn't seem to be the case from the diagrams I've seen. Do you know anything about that?
What do either one of you think would happen if I run a ground jumper directly from the ground side of both rear heater circuits to a good solid ground and by-pass C104 completely. Would that tell me if the problem is in the ground? The reason I haven't already done that is because I thought the auto shutdown relay was involved lin that ground circuit and I would still get the DTCs if I by-passed it. Again thank you both for the advice. Scott

Frango100 06-14-2014 02:34 PM

Re: O2 sensor problems. Need a master mechanic!!
Hi Scott. The auto shutdown relay gets its ground via the PCM, however it goes through the same connector C104, only via an other pin.
If the autoshutdown relay would not activate due to a bad wire, then the engine will not even start, since ignition and fuel injector power are switched via the auto shutdown relay.
Running a separate ground for both O2 sensors seems to be a good idea. If then ok, then you are sure that the problem is in the ground wire and most probably in the splice S146, or connector C104.
Edit: i just see that the front O2 sensors use the same splice S146 and thus also the same connector C104. Since the front sensors are ok, the problem seems to be in the splice S146, or the wiring from the sensors to the splice.

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