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ZZDaryl, Chevy Technician
Category: Chevy
Satisfied Customers: 12549
Experience:  28 year experience, Master Mechanic
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Chevrolet s-10 Blazer: I have a Blazer that has had an intermittent

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I have a Blazer that has had an intermittent problem with the O2 Heater circuit on the left bank (Driver's side) upstream O2 Sensor (P0135). It has progressively gotten worse so I've started diagnosing the circuitry. I couldn't find any problem with continuity on the light green or light green/ white striped wires that power the heater in the O2 sensor.

I decided to pull the wiring harness to visually inspect the wires. I found one of the ground wires that attaches to the rear of the driver's side cylinder head was just about severed through. There is 2 wires that attaches at the back. One is solid black and the other is black w/ white stripe.

The solid black one is the one almost severed through.

Could this be the source of my problems with this circuit or should I continue on?
Hello and thank you for trusting me with your question. My name is XXXXX XXXXX I will do my best to help you with your problem. Yes it is very likely that is the cause of your problem, I just looked at a wiring diagram and that black wire is the ground for the oxygen sensor yoyu are having a problem with, and if it is nearly cut off then that is enough to cause a lot of resistance in that ground and that will cause problems. Let me know if you have more questions on this I am always willing to help, Thanks.
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Customer: replied 4 years ago.

That's what I figured, but just didn't have the resources to confirm.


Thanks Bub!

No problem, thanks for asking

I am having a issue uploading this info and in the time I was trying to do this I figured out I may have gave you misinformation. The black wire to the back of the head us only a ground for the shielding for the wiring for the oxygen sensor. All of the other wires from the sensors go to the PCM. high and low reference for the computer, and you are correct that they do share a ground to the PCM so it may be possible that the ground wire to the PCM has a issue, that wire is light green.
Customer: replied 4 years ago.

Ok, see what you can do about the upload and I'll continue pulling the rest of the harness up to inspect. I was awfully hopeful that would answer the problem, but after thinking it through that these are sharing the same circuit (lt. green/white & lt. green) I was jumping the gun.


Any suggestions are welcome. If you've seen this problem before and could advise accordingly, it would be appreciated.


Thanks again.


Ok let me see what I can do.