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?