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Old 07-07-2012, 08:36 AM   #1
gromittoo
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P0140 rear O2 sensor stays at center

I have a 98 neon DOHC that has just over 100k miles on it. I only drove 3200 miles last year so I got an exemption on PA emissions. It keeps posting code P0140, which basically states that the rear o2 sensor stays stuck at the middle of the range. It like to fix it to make sure I don't damage the CAT, and to make sure that if the check engine light comes on for another reason, I do something about it.

I bought a new Bosch senor on eBay, and I could not get the old sensor off. I took ti to my local mechanic, and they had a bear of a time getting it off. Apparently the worker on the assembly line failed to put anit seize on it. My mechanic got the new one installed.

I took a 50 mile trip, and as soon as I accelerate onto the interstate, the light comes on with the same code. It is possible that the sensor is bad, but I don't think that is the real cause.

I took my neon back to my mechanic when my one of the rear brake cylinders broke in half on that trip (note to others replace the wheel cylinders when you do these rear brakes). I asked him to look at the p0140 code as well.

When I picked up the car, he attached his craftsman scan too, and it showed the voltages. He started the car and let it idle. Strangely, sensor 2 (rear cylinder) stays solid at exactly 4.97 volts, while sensor 1 stays close to 5 volts, but varies a little bit.

My mechanic said his junior mechanic had started checking the fuse box. He wiggled something, and signal came back. That mechanic was not working that day, so we don't have details.

A loose connection would make some sense. Every now and then, all the gauges do dead while driving. I wiggle the key in the ignition, and it sometimes comes back. If it doesn't pulling over and turning car off then back on always works.

Oh and I can pull the key out of the ignition while the car runs. My wife bought the car new, and it has never been broken into or stolen.

I also see something strange where the left turn signal sometimes gets connected to the brake light (I press the brake, and the left turn signal indicator comes while the brake light). Wiggling the turn lever fixes this temporarily. It happens when canceling a right turn signal.

Can Anybody out there explain the circuit that feeds the O2 sensor, and where a bad connection might cause a constant 4.97 volts?
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Old 07-10-2012, 09:13 AM   #2
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Re: P0140 rear O2 sensor stays at center

The following link explains the possible cause as a wiring problem.

http://forums.neons.org/viewtopic.ph...39646#p2739646

Last edited by gromittoo; 07-10-2012 at 09:17 AM. Reason: wrong link
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Old 07-10-2012, 10:16 PM   #3
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Re: P0140 rear O2 sensor stays at center

Quote:
Originally Posted by gromittoo View Post
The following link explains the possible cause as a wiring problem.

http://forums.neons.org/viewtopic.ph...39646#p2739646
Sounds very much like a wiring issue, however alos do not rule out an exhaust leak just before the sensor. I have seen this cause the O2 sensor not to read correctly. Good Luck
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Old 07-11-2012, 07:20 AM   #4
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Re: P0140 rear O2 sensor stays at center

Thanks...

But I am still pretty convinced it is a wiring problem. I borrowed a better ODBII tool, and I can now see the voltage on both O2 sensors while I drive. I drove 25 mile on a twisty road varying speed frequently from 25 to 50 MPH and lots of stops. The the upstream sensor moved from 0.3 to 0.8 volts. The downstream sensor did not budge from 0.470 volts.

I have found the TaN / WHite sensor wire where it goes into the PCM. I am confused about finding the signal ground BlacK / Light Blue.

I will have to jack up the car to connect a bypass wire to where the sensor connector meets the harness. With an ohmmeter, I should be able to ohm it out
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Old 07-11-2012, 09:05 AM   #5
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Re: P0140 rear O2 sensor stays at center

The gauges on the cluster is a common issue and you just need to resolder the connections on the back of the cluster.

The signal light activating when you press on the brake is common as well. The brake light wiring runs thru the turn signal wiring as well. You have a "defective" switch now. You can press on the turn signal slightly to turn the light off or swap in a new/used one.

Your code is definitely a wiring issue. Remember the codes are triggered by a loss in signal or electrical issue most times. They don't mean the part itself is bad.
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Old 07-12-2012, 09:01 AM   #6
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Re: P0140 rear O2 sensor stays at center

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The gauges on the cluster is a common issue and you just need to resolder the connections on the back of the cluster.

The signal light activating when you press on the brake is common as well. The brake light wiring runs thru the turn signal wiring as well. You have a "defective" switch now. You can press on the turn signal slightly to turn the light off or swap in a new/used one.

Your code is definitely a wiring issue. Remember the codes are triggered by a loss in signal or electrical issue most times. They don't mean the part itself is bad.
Thanks for the confirmation. The other night, I was driving with my wife, and she noticed that the speedometer said 5MPH when we were obiously going 40+ MPH. I noticed that ALL my gauges were zero. I wiggled the key, and they came back. I have seen posts about how to do the re-solder thing. I will do that as well.
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Old 07-12-2012, 09:10 AM   #7
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Re: P0140 rear O2 sensor stays at center

I do have a good used turn signal stalk for sale if you are interested. $15 shipped to you.
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Old 07-12-2012, 04:58 PM   #8
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Re: P0140 rear O2 sensor stays at center

I found the culprit. I bought the o2 sensor that had the factory connector on it. The mating connector that is part of the wiring harness has seriously frayed wires, and the signal wire (tan) is completely frayed. I can't fix it with a splice, there is too little wire coming out of the connector. I am going to have to buy a new connector. I'll post a picture tomorrow.
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Old 07-15-2012, 08:36 PM   #9
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Re: P0140 rear O2 sensor stays at center

Yep, a common issue on an older car that has the wires exposed to the environment.
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Old 07-16-2012, 08:18 AM   #10
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Re: P0140 rear O2 sensor stays at center

Here is the photo. My mechanic says just bypass the connector, that O2 Sensor is unlikely to be replaced, and the connector is going to be hard to find and expensive.

He is willing to do it for free, since I paid his shop to replace the O2 sensor (even though I brought the part). Honest mechanics like him are getting harder to find.
Attached Images
File Type: jpg Frayed-O2-Wire.jpg (53.3 KB, 4 views)
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Old 07-16-2012, 08:28 AM   #11
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Re: P0140 rear O2 sensor stays at center

Why can't you just replace it again?
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Old 07-17-2012, 06:02 AM   #12
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Re: P0140 rear O2 sensor stays at center

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Originally Posted by das2123 View Post
Why can't you just replace it again?
The frayed wire is on the wiring harness side of the connector.
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