1995 Blazer service engine light on

[awbooth]

I have a 1995 Chevy Blazer LT 4.3 vortec, 4X4. THE SERVICE ENGINE LIGHT HAS BEEN ON FOR SOME TIME NOW BUT THE TRUCK RUNS GOOD. I had it diagnosed and found a P0137 TROUBLE CODE, DOWNSTREAM HEATED O2 sensor circuit low voltage(bank1, sensor 2). I went ahead and replaced the 02 sensor located after the converter opposed to the one before the converter, had the codes cleared and the service engine light went away, for a day anyway. Ofcourse the light came BACK on again, had it diagnosed again, only to find out the same trouble (P0137) showed up again. Note, the 1st(Upstream O2 sensor) has the proper GM wireharness plug/unplug clips. The one I had to replace had the GM wire harness missing and it was hard wired in. Looks like some one tried to put in a new one in the past. In any event, low voltage? Maybe the sensor I replaced was just fine to begin with? Please help? Thanks
Anthony
[email protected]

[GMMerlin]

Most likely the sensor was wired incorrectly or there is a bad connection.

[awbooth]

Originally Posted by GMMerlin

Most likely the sensor was wired incorrectly or there is a bad connection.

Thanks for the reply. I went ahead and looked at the upstream O2 sensor and saw exactly were the 4 wires go, on GM side of harness I have a tan, purple, black and pink. The O2 sensor has a grey, black, whit and white wires. The downstream sensor has the same 4 colors even though there is no harness pigtail. I used Connect/disconnect terminals, used proper crimping tool, and made sure all connections were tight. The light still came on, I just recently purchased a new O2 sensor, I'm going to leave the harness on there and go to GM and order the proper female plug in pigtail and then wire it. I still think though the light will turn back on again. Do you think the converter is shot and the emissions coming out of the pipe is setting off the downstream sensor? Or better yet how about improper fuel/air mixture, the sensor senses something wrong and sends a signal to the computer? I'm just guessing. Thanks, let me know what you think.
Anthony

[RABarrett]

More than likely, the O2 sensor is not being properly heated, and the computer is ticked that the voltage does not rise fast enough. It is also possible that the incorrect wiring damaged the computer driver, which heats the sensor. The computer watches the voltage on both the heater circuit and the sensor output. If the sensor heater voltage stays low, or high all of the time, it will set a heater code. If the voltage from the sensor does not operate correctly, it will set a sensor code. After properly installing the harness, check the circuit. If all is well, the light will not reilluminate. The likelyhood of computer damage is likely here. If necessary, I can walk you through that. Ray

[GMMerlin]

The Pink wire is the 12v feed for the heater..should be position D on the connector
Black wire is the heater gound..position C on the connector
Purple wire is the sensor high signal to the PCM..position B on the connector
Tan wire is the sensor low to the PCM..position A on the connector
Make sure you have 12v on the Pink wire with the key on/engine off...if not, check the fuse.
Once you wire in the new connector and verify the wire positions, you can continue with your diagnostics.

[awbooth]

Originally Posted by RABarrett

More than likely, the O2 sensor is not being properly heated, and the computer is ticked that the voltage does not rise fast enough. It is also possible that the incorrect wiring damaged the computer driver, which heats the sensor. The computer watches the voltage on both the heater circuit and the sensor output. If the sensor heater voltage stays low, or high all of the time, it will set a heater code. If the voltage from the sensor does not operate correctly, it will set a sensor code. After properly installing the harness, check the circuit. If all is well, the light will not reilluminate. The likelyhood of computer damage is likely here. If necessary, I can walk you through that. Ray

Ok. Thanks alot for emailing me. It's great to write about this to people who have knowledge and skill that can help. There is a strong chance the computer is damaged. I'll tell you why. I'm on my second sensor making sure that the 4 wires from GM side, Tan, purple, black and pink go as follows, Tan wire from GM side joined to Grey on sensor side, Purple from GM side joined to Black wire on sensor side, Black wire on GM side joined to white on sensor side and Pink joined to white on sensor side. Now before I cut the clip off the new sensor I unplugged the upstream sensor (which has the proper GM HARNESSon both sides) and matched the colors and how the plug plugs in. I did that to be triple sure that the 2 white wires don't get mismatched between the pink and the black on GM side. I have gone through every way to install it and I'm convinced you are right about the computer only because I'm using an Actron 9135 OBDII scan tool, checking codes, clearing codes etc. etc. and I'm still getting the same P0137 heated o2 sensor low voltage? Is there a chance that the emissions coming out of the exhaust is bad or to rich, to lean.,or a bad converter which causing the sensor to send a signal and cause the service engine light to turn on? I can tell the cat and the exhaust and pipe looks brand new, you can tell when there is age on exhaust. In any event I would appreciate the walk on the computer, please, thanks alot Ray.
Anthony

[awbooth]

Originally Posted by GMMerlin

The Pink wire is the 12v feed for the heater..should be position D on the connector
Black wire is the heater gound..position C on the connector
Purple wire is the sensor high signal to the PCM..position B on the connector
Tan wire is the sensor low to the PCM..position A on the connector
Make sure you have 12v on the Pink wire with the key on/engine off...if not, check the fuse.
Once you wire in the new connector and verify the wire positions, you can continue with your diagnostics.

Ok. Thanks for your email. I appreciate your effort and the effort of others in helping me. I ordered the wire harness for the GM side of the truck but it was a special order. I went ahead and cut off the harness clip on the sensor and hard wired it in. I used the proper crimping tool and the connector but ends to connect the GM side and the Sensor side. I went exactly by your directions and after driving for a short period of time the service engine light came on with the same P0137 trouble code. As long as I wired it properly, shouldn't it be sufficent? Is is a must that they are plugged in with the harness clips? The upstream O2 sensor has the wireharness plug ins, they didn't cut those off, just the downstream one and if they wired it wrong, could they have done damage? I made the connections tight, covered with electrical tape and made sure it would stay dry. It hasn't rained anyway so rain is not a factor? I don't know what to think? Bad emissions and the sensor is going off becasue of that? Bad converter? It's just a stupid light but I want to fix it right, I'm some what of a perfectionist(pain in the ass) but I'm trying my best to rid of this service engine light, by the way, the blazer runs perfect in everyway. Let me know what you think.
Thanks again.
Anthony

[Wildcatt88]

Hey, just wanted to say I have a 95 Blazer 4x4 also. They's a trip! Nice truck, but I have had a ton of trouble with the emmissions on the darn thing!

[GMMerlin]

At this point in time and with everything you have done, I would lean towards replacement of the PCM

[awbooth]

Originally Posted by GMMerlin

At this point in time and with everything you have done, I would lean towards replacement of the PCM

I've been trying to find a PCM. I'm not sure where I can get one. The dealer prices are brutal, $400.00 for the computer and an addtional $75.00 to "flash it" or program it. Shoot me now, please! Thanks GM Merlin
Anthony

[GMMerlin]

Check www.gmpartsdirect.com they may have it.