Topic: O2 sensor mystery

[Complete Loser]

Well, i have only workd on OBD2 vehicles, or non OBD all together. So i could be missing something here but...

when i got my jeep, i noticed that the O2 sesor wired were cut at the sensor.
No biggie. I figured i would just buy another one and plug it in and go.

Well...... i CANT FIND THE WIRES ANYWERE!   We have looked and looked, Short of pulling the harness, we cant find those specific wires.

To make matters more confusing/mysterious, i have made several drive cycles and the check engine light never comes on. (no its not burnt out).

When the check engine light does come on, it never lists an O2 code. Such as if i force the light on by unplugging the TPS or something.

Any thoughts on this one? 
WTF?

I would like to use the O2 for best fuel mileage.

[Jeffy]

What year is the Jeep?  Is this a pre OBDI or a OBDI?

[sharpxmen]

Well, i have only workd on OBD2 vehicles, or non OBD all together. So i could be missing something here but...

when i got my jeep, i noticed that the O2 sesor wired were cut at the sensor.
No biggie. I figured i would just buy another one and plug it in and go.

Well...... i CANT FIND THE WIRES ANYWERE!   We have looked and looked, Short of pulling the harness, we cant find those specific wires.

To make matters more confusing/mysterious, i have made several drive cycles and the check engine light never comes on. (no its not burnt out).

When the check engine light does come on, it never lists an O2 code. Such as if i force the light on by unplugging the TPS or something.

Any thoughts on this one? 
WTF?

I would like to use the O2 for best fuel mileage.

so is this a 91-95 MPFI?

if the CEL is functioning properly it should come on if there is no o2 signal - can you take some pics so we can see the cut wires in question?

[Complete Loser]

Sorry about that. I thought my jeep info was in my profile. (used to that 'other'  forum) Stock 92' yj.

Not sure if a pic would help. Its the sensor, with about 4 inches of wire hanging out of it, cut.
The other end (the harness end that goes to the O2) i cant find anywhere.

I appreciate the replies.

[sharpxmen]

Sorry about that. I thought my jeep info was in my profile. (used to that 'other'  forum) Stock 92' yj.

Not sure if a pic would help. Its the sensor, with about 4 inches of wire hanging out of it, cut.
The other end (the harness end that goes to the O2) i cant find anywhere.

I appreciate the replies.

maybe you're missing the other side and you have to look only for the connector - it is usually located on a bracket at the level where the oil pan meets the block - it is black and has 4 wires. you can follow the harness that goes between the steering pump and the intake manifold downwards, it is at the end of that (or if it is not at least you'll find the cut wires so you can tie in there with a new connector).

[Complete Loser]

Well, i will have another look. But right now the intake and exhaust manifolds are out of the jeep. So not much there to miss right now. But i will take a fresh look in the mornin.
There is what looks like an old coolant sensor attached to a freeze plug on that side of the block. No wires going to what ever it is either. Its in the back most freeze plug.

I gave a good hard look at the power steering plug. But ofcourse, there wasnt even enough wires there.

[sharpxmen]

There is what looks like an old coolant sensor attached to a freeze plug on that side of the block. No wires going to what ever it is either. Its in the back most freeze plug.

that is probably a plug-in block heater - i have one, still wired but never used it - it is wired with a 120v plug, that's probably what it is and you're missing the wiring for it.

[Complete Loser]

Looked again. I found the bracket, but cant find anything else. In the pic, you can tell i trimmed the wires on the sensor even further.

[sharpxmen]

take a pic on top of the engine where the harness splits to go to the thermostat housing where there's a temp sensor - the other leg goes downwards to the steering pump pressure switch and the o2 sensor. I'll see if i can find a pic that shows that.

[Complete Loser]

Will do buddy. :)   That does start to make sense. Because the ONLY spare wire i have found, is a lone gray wire. It is very short (been hacked on) and it is in the general loom area of the power steering switch.
hmmmmmm.....

[sharpxmen]

Will do buddy. :)   That does start to make sense. Because the ONLY spare wire i have found, is a lone gray wire. It is very short (been hacked on) and it is in the general loom area of the power steering switch.
hmmmmmm.....

there's no gray wire on the O2 sensor

this is what you should have

Dark Green with Orange stripe - o2 heater 12v supply off the ASD relay
Black - heater ground
Black with Light Blue stripe - sensor ground
Black with Dark Green stripe - o2 sensor signal to PCM.

you should be able to follow the Black/Dark Green wire downwards thru the harness as the arrow shows in this pic and see where they are cut - you'll need to get that end of the harness and solder the wires in if they are indeed been chopped off

[Complete Loser]

I tore open the harness in that area, and sure enough, there they are!

However, my chilton says i need a gray, black, brown/red, and dk green/black wire.

And that appears to be the ones ive found.



Awesome help! I wanted to tear into the harness, but didnt know what lead the o2 ran from.
Hopefully these are them.
Once i get this thing running better, i will probe them and verify.

[sharpxmen]

I tore open the harness in that area, and sure enough, there they are!

However, my chilton says i need a gray, black, brown/red, and dk green/black wire.

And that appears to be the ones ive found.
[...]
Awesome help! I wanted to tear into the harness, but didnt know what lead the o2 ran from.
Hopefully these are them.
Once i get this thing running better, i will probe them and verify.

you're probably right about the colors, the ones i listed were the 94/95 MPFI. glad you found them. make sure you don't mix the sensor ground and heater ground (the heater ground is thicker 16ga, the sensor ground supposed to be 18ga).