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Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4764
Experience:  ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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2002 Blazer 4.3 Vortec. Had codes for o2 sensors bank 1 #1

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2002 Blazer 4.3 Vortec. Had codes for o2 sensors: bank 1 #1 and bank 2 #1. Replaced both with exact fitting Bosch sensors (no splicing). Cleared codes.

Now getting 2 codes: P0135 P0155 heater circuits for bank 1 sens 1 and bank 2 sens 1. - the same 2 I just replaced. Since the other O2 sensor is not throwing a code It is not the wiring - I also checked the fuse which is intact.

Oh - one last thing - on start up after it's sat awhile it will hesitate when you start driving. This lasts a minute and goes away (assuming when the o2 sensors heat up?)

- help!
I'm not convinced that the hesitation is related to the oxygen sensor issue. It may be, but for the purposes of this discussion, let's ignore that for now.

Those codes relate to the heater circuits only for those two sensors. The diagnosis is easy: disconnect the sensor, put a test light to the heater circuit pin on the connector, and turn the key on. The test light should be lit. If it's not, then you need to find out why.

Those two sensors receive the power and the ground for the heater circuits from the PCM itself. The other O2 sensor has a separate fused heater circuit.... don't ask me why, I don't know. But check the LTGRN/WHT wire for power on each, and check the LTGRN wire for ground, and trace those back to the PCM if you find that either of them doesn't have the necessary reference.
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Customer: replied 7 years ago.
Ok going to check it out today...
Customer: replied 7 years ago.
Definately shorts in the heating section. Found frayed wire near the computer. Fixed but still had issue. Traced it down to where it splits off to each sensor. Cut the split and patched just to the passengers side. It's good now. The shortsos between where the lt gen/wht stripe line splits and the drivers side o2 sensor. Note it's not the 02 sensor as I tried the line with it disconnected.

Hard to reach behind the engine to trace it. Any ideas where the fault might lie?

I'm considering cutting the wire at the drivers 02 sensor and running a seperate line...

Customer: replied 7 years ago.
I think I know which wire is crossed with the lt grn/wht srtipe line of the drives side O2 sensor. The only other thing that is flaky in the car is the gas gauge.

I am guessing the wiring to the gas gauge runs through the same harness as the drives O2 sensor - am I correct?

What's the color wire for the gas gauge?
Well, not exactly. There is a purple wire that goes to the fuel level sensor, that reports the fuel level to the PCM. The PCM then reports the fuel level to the instrument panel over a class 2 data bus.

However, I seriously doubt that the oxygen sensor issue is related, particularly because Blazers are notorious for fuel gage problems, stemming from faulty fuel level sensors.
Customer: replied 7 years ago.
oh well. I guess you are right. Anyway I have a new fuel pump assembly going in next week when I have access to a lift.

But back to the O2 sensors - so 2 days later and the Check Engine light is back on. I ran the code and it is Bank 1 #1 - the drivers side sensor that has the disconnected heater (remember I cut the lt grn/wht stripe line to that sensor. Glad to see bank 2 #1 that I re-connected is working properly now...

Anyway considering that both bank 1 #1 and bank 2 #1 ran from the same original wire from the computer - how did the computer know one was disconnected? Because of the ground line???

I'm going to cut the lt grn/wht stripe line right before the bank 1 #1 drivers side O2 sensor and run a new line. As long as the overall issue wasn't with the ground line I assume everything will then be fine with the oxygen sensors...

sound right to you?
The PCM tests the heater to see if it works by just watching the O2 voltage on startup. If the heater works, the voltage will slowly drop on the sensor, and the PCM anticipates that. When it doesn't drop, the PCM assumes that something is wrong with the heater and sets a code. Pretty simple.

Running a new wire is the way to go on stuff like this. Just make sure you use some good splice connectors that are weather tight.
Customer: replied 7 years ago.
thanks again for the help. I'll let you know how it goes!
Customer: replied 7 years ago.
Progress - well sort of...

So ran a test line from the lt grn/wht strip at the computer to the drivers side (bank 1 #1) oxygen sensor. Immediately got the wrong voltage again to both bank 1 sensor heaters. So now I have to suppose the short is actually in the second heater line and not the lt grn/wht stripe.

Which line is the second heater line at the drivers side sensor (bank 1 #1) and where can I find it at the CPU?
Customer: replied 7 years ago.
found a pinout of the computer.

Red plug:

#72 brown wire - o2 heater low control
#74 lt green/white - o2 heather high control (to Bank 1#1 & Bank 2#1
#80 lt green - 02 heater low reference

Which wire #72 or #80 is the correct one to splice to the drivers o2 sensor bank 1 #1?
Pin 80 is the correct one.
Customer: replied 7 years ago.
Awesome. I will try that tomorrow. Hopefully all will be fine after that. Thanks again for the help.
Customer: replied 7 years ago.
Hi again. I am officially perplexed.

There are only 2 wires from the computer to both number 1 o2
sensors? Both split about 12" after the come out of the computer.

The pin 74 - lt grn/wht wire is fine - 12v.
The pin 80 - I spliced a new wire from bank 1 number 1 to right after
the wire splits by the computer.

Just like when I tried bypassing the lt grn/white stripe the check engine
light went off for a day and now is back on.

Same code 0135 bank 1 sensor 1 heater.

How is this possible? Bad computer? Defective o2 sensor?
How does the computer know which o2 sensors heater is bad if both
bank 1 and bank 1 heaters run on the same 2 wires from the computer?

Sorry this is dragging on. I just need this thing to pass inspection already!!!

Thanks again. Scott
My guess would be defective O2 sensor. Like I said earlier, the PCM is watching the O2 sensor voltages (not the heater voltages, the feedback voltages) and expects the voltage to drop as they heat up. That's how the PCM surmises that the heaters are working, or not in this case.

Let me see if I can attach the schematic, so perhaps you can understand the layout better:

Customer: replied 7 years ago.
Ok next up I'll swap 2 sensors and see if the code follows it to the new location...
Customer: replied 7 years ago.
Exchanged the o2 sensor and going on 2 days now with no check engine light!!!!

Thank again for everything!!!

Next up:
trouble starting cold and air bag light on intermittently

(I'll pay for more help)

my thoughts: fuel pump for the rough start and clock spring for the airbag light
what do you think?
Definitely thinking fuel pump on the starting concern. Airbag could be a lot of things, probably the one thing I see most often is front end sensors that go bad.

Both require a bit of diagnosis, obviously, I'm just guessing in the dark. Glad the O2 thing is going well for you!