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Old 05-11-2012, 02:29 PM   #1
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Ok so a lot of posts about o2 sensors and reminded me I need to ask about the downstream sensor previous owner did a custom exhaust and stupidly cut the hook up to the o2 sensor and now I don't kno wht it looks like can someone post up pics of the wire for the rear o2 sensor there is a cut harness I found but don't know if it's it and how would I hook up the sensor back up sense it's cut from the plug


Also iv read that the rear o2 sensor doesn't do anything and it doesn't hurt anything while unhooked aside from the CE light

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Old 05-11-2012, 02:55 PM   #2
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Also iv read that the rear o2 sensor doesn't do anything and it doesn't hurt anything while unhooked aside from the CE light
I don't buy into that internet myth.

If you read the Factory Service Manual, it does suggest the downstream indeed has an effect. It's job is to monitor the "health" of the catalytic converter(s). My theory: It monitors this by cycling between rich and lean conditions...so the upstream O2 sensor sets the A/F ratio, but the downstream will trim the fuel around that, in order to keep the cats running at optimum efficiency. The cats need to run within a specific temperature range...too cold, they do not convert greenhouse gasses...too hot, the internals melt. The PCM can keep them within the optimum temperature range by adding or removing a little bit of fuel. Slightly rich - the cats run hotter, slightly lean (or stoich) - the cats run cooler. It does this by monitoring the oxygen leaving the combustion chamber, and by comparing the upstream value with the downstream value. If there is the same oxygen at the upstream location as the downstream location, the PCM knows something is wrong. For example, it may think the cat is not working properly....so you get an code for a catalytic converter efficiency. It may even try to heat up the cat by running rich...well if the cat isn't there, you get a CEL and bad mileage. Additionally, the PCM is expecting a specific voltage reading back from the sensor based on what other sensors are doing, so if the downstream O2 sensor is unhooked, you get a code for a O2 sensor fault because the PCM is not seeing any voltage coming back from the sensor. Because the PCM can't tell what's happening, or what the temperature or status of the cat is, it will default to running rich, which is safer than running lean. So your fuel mileage goes in the toilet, and a CEL is displayed to tell you know something isn't operating correctly.

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Old 05-11-2012, 03:19 PM   #3
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Originally Posted by UnlimitedLJ04

I don't buy into that internet myth.

If you read the Factory Service Manual, it does suggest the downstream indeed has an effect. It's job is to monitor the "health" of the catalytic converter(s). My theory: It monitors this by cycling between rich and lean conditions...so the upstream O2 sensor sets the A/F ratio, but the downstream will trim the fuel around that, in order to keep the cats running at optimum efficiency. The cats need to run within a specific temperature range...too cold, they do not convert greenhouse gasses...too hot, the internals melt. The PCM can keep them within the optimum temperature range by adding or removing a little bit of fuel. Slightly rich - the cats run hotter, slightly lean (or stoich) - the cats run cooler. It does this by monitoring the oxygen leaving the combustion chamber, and by comparing the upstream value with the downstream value. If there is the same oxygen at the upstream location as the downstream location, the PCM knows something is wrong. For example, it may think the cat is not working properly....so you get an code for a catalytic converter efficiency. It may even try to heat up the cat by running rich...well if the cat isn't there, you get a CEL and bad mileage. Additionally, the PCM is expecting a specific voltage reading back from the sensor based on what other sensors are doing, so if the downstream O2 sensor is unhooked, you get a code for a O2 sensor fault because the PCM is not seeing any voltage coming back from the sensor. Because the PCM can't tell what's happening, or what the temperature or status of the cat is, it will default to running rich, which is safer than running lean. So your fuel mileage goes in the toilet, and a CEL is displayed to tell you know something isn't operating correctly.
Ok thanks for clearing that up it does have a cat but the PO got a universal one and didn't get the bung for the o2 so screw into so he cut the sensor off from the clip and the CEL is on and want to weld a bung on and put the o2 back in but I don't know what the wires look like since the clip was cut off so would u happen to have a pic of ur o2 harness or plug My tj is a 98 4.0
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Old 05-11-2012, 03:28 PM   #4
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Ok thanks for clearing that up it does have a cat but the PO got a universal one and didn't get the bung for the o2 so screw into so he cut the sensor off from the clip and the CEL is on and want to weld a bung on and put the o2 back in but I don't know what the wires look like since the clip was cut off so would u happen to have a pic of ur o2 harness or plug My tj is a 98 4.0
is the O2 sensor plug still there? there should be a female end and a male end. If the end which is connected to the PCM is still there, you're fine. Just buy a new downstream O2 sensor, weld the bung directly behind the cat and plug-n-play.

If the connector end which is connected to the PCM is missing, you will need to buy a factory service manual (FSM), a new connector (junkyard maybe?), splice it in based on the FSM wiring diagram, then install the new O2 sensor. You may need some heat sheild/insulated wire loom or something as well to keep the wires from melting.
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Old 05-11-2012, 03:36 PM   #5
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is the O2 sensor plug still there? there should be a female end and a male end. If the end which is connected to the PCM is still there, you're fine. Just buy a new downstream O2 sensor, weld the bung directly behind the cat and plug-n-play.

If the connector end which is connected to the PCM is missing, you will need to buy a factory service manual (FSM), a new connector (junkyard maybe?), splice it in based on the FSM wiring diagram, then install the new O2 sensor. You may need some heat sheild/insulated wire loom or something as well to keep the wires from melting.
I can't find the female end where it comes off the PCM ill post pics in a sec of underneath and of the cut wires
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Old 05-11-2012, 04:05 PM   #6
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This was the only was the only unplugged wire

And this was the cut wire
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Old 05-11-2012, 04:49 PM   #7
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Here's a pic wit the tape taken off I think this is it
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Old 05-11-2012, 04:56 PM   #8
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those four wires that the moron cut are indeed for the oxygen sensor. trace them back and see if they attach to a plug somewhere off camera. you will likely need to get a 1998 FSM and the plug end for that side of the harness, then reinstall the O2 sensor.
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Old 05-11-2012, 05:13 PM   #9
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those four wires that the moron cut are indeed for the oxygen sensor. trace them back and see if they attach to a plug somewhere off camera. you will likely need to get a 1998 FSM and the plug end for that side of the harness, then reinstall the O2 sensor.
Does the wire run front the PCM to o2 sensor or is there another plug all I see it coming out of a bunch of other wires should I find one at a junk yard and splice it in?
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Old 05-11-2012, 06:10 PM   #10
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Does the wire run front the PCM to o2 sensor or is there another plug all I see it coming out of a bunch of other wires should I find one at a junk yard and splice it in?
the wiring harness from the PCM will come down and branch out into multiple items, all of which have individual plugs. the O2 sensor has it's own individual plug, and each O2 sensor comes with it's proper plug pre-installed on the necessary length of wire. its just plug-n-play....if you still have the plug on the PCM end.
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Old 05-11-2012, 06:18 PM   #11
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the wiring harness from the PCM will come down and branch out into multiple items, all of which have individual plugs. the O2 sensor has it's own individual plug, and each O2 sensor comes with it's proper plug pre-installed on the necessary length of wire. its just plug-n-play....if you still have the plug on the PCM end.
Ok I'll have to look where can I buy a plug at a regular parts store? Along with the the o2 sensor
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Old 05-11-2012, 06:24 PM   #12
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Ok I'll have to look where can I buy a plug at a regular parts store? Along with the the o2 sensor
the O2 sensor can be bought anywhere that sells most autoparts.

try RockAuto Auto Parts

the plug itself, I have no idea. That will probably be something you must figure out yourself. I'd suggest checking local junkyards first. I suspect most Chrysler vehicles likely use the same connectors, I'd start by looking at XJ's and ZJ's of your same year. Basically find the plug and cut it off.
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Old 05-11-2012, 06:40 PM   #13
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the O2 sensor can be bought anywhere that sells most autoparts.

try RockAuto Auto Parts

the plug itself, I have no idea. That will probably be something you must figure out yourself. I'd suggest checking local junkyards first. I suspect most Chrysler vehicles likely use the same connectors, I'd start by looking at XJ's and ZJ's of your same year. Basically find the plug and cut it off.
Ok I'll have to go to the junk yard and look around also I found these to plugs unplugged should I plug them or leave them idk what they do the female end is coming out the transmission it looks like
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Old 05-13-2012, 05:37 PM   #14
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Alright so I took unlimiteds advice and went to the junk yard about found some plugs out of a cherokee 4.0 so I'm looking at the pics of mine and the color of the wires arnt the same how can I splice them and put them together
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Old 05-13-2012, 05:43 PM   #15
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Alright so I took unlimiteds advice and went to the junk yard about found some plugs out of a cherokee 4.0 so I'm looking at the pics of mine and the color of the wires arnt the same how can I splice them and put them together
you need a factory service manual to see the wiring diagram, so you know what to match up. I know the two white wires are the heater circuit on the O2 sensor, but that's all I can tell you. Get a FSM so you know what you have to do.
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Old 05-13-2012, 05:45 PM   #16
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you need a factory service manual to see the wiring diagram, so you know what to match up.
Ok so will those Manuals at the parts store be good or I have to get a specific one
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Old 05-13-2012, 05:52 PM   #17
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Ok so will those Manuals at the parts store be good or I have to get a specific one
the auto parts store manuals are crap.

Again, you need a Jeep Factory Service Manual. It is specific to your year/make/model.
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Old 05-13-2012, 05:54 PM   #18
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the auto parts store manuals are crap.

Again, you need a Jeep Factory Service Manual. It is specific to your year/make/model.
Where could I pick one up at?? The dealership? eBay?
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Old 05-13-2012, 05:57 PM   #19
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Where could I pick one up at?? The dealership? eBay?
type your year/make/model with "factory service manual" into google and see what happens. lots of options.
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Old 07-03-2012, 03:10 AM   #20
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So the 2000 TJ I just bought had the downstream O2 sensor in open air and the bung closed with a plug.
It did send a code for the sensor being over voltage.
I plugged the sensor back in but have yet to see if the code will stay gone now.
Why did somebody do this?

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