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2003 chevy trailblazer_lt
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Discussion Starter · #1 ·
I had the code P0420.
I couldn't remove the O2 sensor-and broke the wires off the original o2 so I had a shop do it for me.
Upon installing the replacement, now I get a different error- P0141 (o2 heater not working).

I thought perhaps it shorted the fuse when the wires broke off. I looked at fuse 29 but it is fine.
Can anyone tell me what fuse it is if not 29? or did I get a bad o2?
Lastly, I need to look if I still have it but if I have the old wire harness, what wires to test if there really is voltage?
TIA.
 

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2004 chevy trailblazer_lt
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Which engine?
 

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2004 chevy trailblazer_lt
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Thanks.

Fuse 29 is the fuse. You might want to swap it with another fuse of the same rating. If it still throws a code, then you need to use a digital multi meter (DMM) and using the ohms function, see if there is continuity between the ECM and the downstream O2 Sensor's connector.

You want to check between O2 sensor connector pin D and both sides of fuse 10 in the underhood electrical center.

You also want to check between O2 sensor connector pin C and Pin 30 of Connector C2 at the PCM.

Attached is the schematic for the O2 sensors circuits. What is strange is that the schematic is showing Fuse Number 10 as the fuse in the 12 VDC going to the downstream heater circuit.

I apologize, but I am unable to find a drawing of the O2 sensor connector.

Hope this helps!
 

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Discussion Starter · #6 ·
I already tried swapping fuses. No dice.

Yes, that is the connector.
I'll grab that voltmeter and see if it is even getting heater voltage.

Thanks Guys!
 

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2004 chevy trailblazer_lt
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Sounds good!

Good Luck!
 

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Discussion Starter · #8 ·
Trivial question but does the heater only work when the engine is cold by chance? When I looked at it, the engine was still kinda warm. I used the old wire and connected a volt meter to the wires and started it up. No voltage. I tried grounding the volt meter to the frame instead of the grounding wire. Still no voltage.
Thanks.
 

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2004 chevy trailblazer_lt
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I believe that the heaters are always on. I think you have discovered your problem. Now to track down where the wire or wires are broken/grounded out/shorted. Good luck!
 

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'05 Chevy TB EXT
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IN older systems --- where I have most of my experiences --- the heaters came on no matter the starting ambient value and would cycle off when the sensor started "talking" to the ECM.
√ At idle, it might come back on to keep the sensor viable for when the vehicle started moving again and --- yup --- it would shut off when the uncleaned exhaust flow was sufficient to keep the cat lit.​

I believe this was a policy to keep use of the heater element to a minimum since they were not terribly trustworthy and had a short cyclic lifespan.
 

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Discussion Starter · #11 · (Edited)
Sounds good!

Good Luck!
I looked at the doc u attached.(Thanks) The circuit diagram says it is always on. Interesting that it shares the same fuse as the second o2...
Oh well, time to crimp on test wires...
Strange...computer only complains on 1 o2 when they share the same power fuse(so assumed get power) unless the problem is really the computer...
 

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The heaters may be on the same 12v heater circuit .... but I find that counterintuitive because the sensor closest to the hot exhaust near the combustion process SHOULD not tend to extinguish as easily. That's my assumption anyway.

I've been wrong before ... once in 1953 .... in 2nd grade when I failed to conjugate the verb "did" correctly in the Future Feminine Pluperfect case ... in Latin.
FTR ---> it was supposed to be: "{She} would have done" and I wrote "{She} would have dood".​

I know it seems funny now .... g'wan an' laugh! ..... but my knuckles got a severe birch rod pummeling ... in front of the whole class too!.
 

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The heaters may be on the same 12v heater circuit .... but I find that counterintuitive because the sensor closest to the hot exhaust near the combustion process SHOULD not tend to extinguish as easily. That's my assumption anyway.

I've been wrong before ... once in 1953 .... in 2nd grade when I failed to conjugate the verb "did" correctly in the Future Feminine Pluperfect case ... in Latin.
FTR ---> it was supposed to be: "{She} would have done" and I wrote "{She} would have dood".​

I know it seems funny now .... g'wan an' laugh! ..... but my knuckles got a severe birch rod pummeling ... in front of the whole class too!.
Your life is beginning to make sense!
 

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Discussion Starter · #14 ·
Both heaters go from the same + fuse but after the heaters, they go to seperate pins on the computer.
I don't feel like finding why the rear heater is not getting power so I ran a + wire to the trouble o2 from the front working o2 and now, checking the pin going into the computer, both front and back o2 send 4V to the computer.

So now, I get the error as if the other two wires don't work. With several wires mysteriously broken, makes me wonder if the shop that took off the o2 just yanked it off breaking the wires.
 

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............................ then you have to run a continuity test from one end of the wires back to the ECM.

Report back.
 

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Discussion Starter · #16 ·
............................ then you have to run a continuity test from one end of the wires back to the ECM.

Report back.
I did this last night.
I used the original o2 and ran wire from it to the old harness connecting to the engine computer.
Thing is I get open circuit. I got a pic that showed the pins but now I wonder if I am looking at the right connector- the middle one...
Being I am just testing for continuity, I connected both o2 wires(old harness) to 1 return wire. Thing is... I forgot to reconnect the new o2 and started the truck. It ran like crap. I immediately remembered. I grabbed teh test wire and yanked it disconneting the short and the truck ran fine since.

So.... in other words, there IS continuity as it was shorting but being I DON"T get continuity with the meter(don't worry I tested it-it's a fluke tester)..
Now I am debating- pay and have a shop hook up a computer to see if indeed the signals are going out to the o2 or just get another one that maybe I got a lemon....?
 

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'05 Chevy TB EXT
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Now it's your call .... your money .... your car.

You are there ... we are here and this is your decision.
 

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Discussion Starter · #19 ·
................ <and???>
I gambled and went to the store and bought one. It was only $50 anyway. I don't get the errors from the previous but I am back to square one in that I get the error P0140??(sorry don't have phone).
I guess it orignally was indeed cat acting up- thing is I bought and replaced it with a brand name one a few years ago...
 

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2004 chevy trailblazer_lt
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Do you mean a P0420 code? Have you checked your long tern and short term fuel trims? Let's see, you only replaced the downstream O2 sensor, correct? Have yo ever changed the upstream one? If not they do get lazy with age and mileage. Might be worth a try.
 
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