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Chevrolet Blazer Check Engine Light On

Q. Hi, Thank you taking my email. I just recently purchase a 1995 Chevrolet Blazer LT 4.3 liter V-6 4X4. I purchased the vehicle with the Check Engine Light on, I figured I can tackle the job myself. I work on my cars and my families cars and I feel good about working on cars. I enjoy doing it, sometimes!

Chevrolet Blazer Check Engine Light On

I had the Check Engine Light diagnosed with an OBD-II Scan tool made by Actron and the tool gave the trouble code P0137, downstream O2 sensor low voltage, Bank 1, Sensor 2. I went ahead and replaced the O2 sensor located after the converter opposite to the one before the converter.

I then had the trouble code cleared by the same scan tool and the light went away, FOR A DAY!!! The next day the light came back on as if I did nothing. The trouble code came back again P0137 low voltage downstream O2 sensor.

I went through a lot of trial and error messing with ECM fuses, moving wires around but I have yet to find the problem. Low voltage? Maybe the O2 sensor never needed to be replaced? What do you think, your thoughts and knowledge would be greatly appreciated.

Oh by the way, the upstream sensor, located before the converter, has the original GM Wire harness that allows easy plug/unplug clips. The sensor I replaced did not have the wire harness. It was hardwired in as if they eliminated the clips and wired in a universal deal or something. In any event I wired up the new sensor the same way the old one was wired, I had to cut the wire harness off the new sensor to hard wire it like it was when I found it.

It is a heated O2 sensor with 4 wires, gray, black, white and white, the GM colors coming from the truck is tan, purple, black and pink. Why does the sensor have two white wires with no idea which is which? In any event I wired the new one the same as the other by creating a wire diagram. The light came back on after clearing codes and replacing the O2 sensor. Please help.


A. Hardwired? That doesn't sound good! There should be a short harness and then a 4 wire black plastic plug on it. If not, this may be your problem. Low voltage means that the ECM is seeing a low voltage from that particular sensor for too long a period of time.

It could mean the car is running too lean (low voltage usually means lean condition). Of the 4 wires 1 is a signal wire, 1 is the reference voltage, the other 2 are for the heater portion. The 02 sensor needs a constant supply of fresh air which it gets from actually pulling it in through the wires.

If someone crimped the harness they closed off the air supply. The GM manual says no repairs to the harness pigtails, these are the wires that come with the sensor out of the bag. Someone may have done this because of corrosion at the original plug area.

There should be a harness running from the PCM down the right side of frame rail that can be purchased as well as another new 02 sensor. It would have the other side of the plug that plugs into the sensor plug.

This would be my first thing to do. After that if the code resets you will need to scan the real time data stream to see when it's going "low" and go from there.

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