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2003 S10 4.3 2WD
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Discussion Starter · #1 ·
2003 S10 4.3 2wd 123,000 miles

So now after getting the fuel pump replaced and fuel tank pressure sensor wiring harness replaced and everything going smooth. Now all of a sudden my truck decided to throw o2 sensor code.

It threw

P0155 o2 Sensor Heater Circuit Bank 2 Sensor 1
P0135 o2 Sensor Heater Circuit Bank 1 Sensor 1

Doesn't seem to be messing with how it runs, I cleared the codes and it hasn't come back yet. But I just want to verify that none of the above sensors are A/F sensors because otherwise if it does come back I will just drive with it for a while until I can get the time to get underneath to see what went wrong,

I am suspecting more wiring damage but will probably just replace them eventually even though my exhaust looks brand new so they probably were replaced before by the original owner. No emissions or inspections in my state so not in any hurry.
 

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non functioning O2 sensors will mess up your AF, and destroy your cats. since it is a heater circuit on both, check your fuses. Also, do a search and see if anyone has reported this issue, ight be an easy fix before replacing the O2s.. also, depending on mileage, it might be a good idea to replace the O2s as they are a wearable item..
 

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Discussion Starter · #3 ·
Is there a ground wire or anything to these o2 sensor under the bed? it is a regular cab long bed 2003. I did the bed off method for the fuel pump and also did rust prevention on the frame.

This truck doesn't seem to have anything marked for an o2 sensor fuse.
 

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Discussion Starter · #4 ·
I just hooked it up to check live data and it is showing 0.0 for both sensors.

There is a 20amp fuse under the hood it says "Oxysen" not "Oxygen" is this the fuse maybe its short for oxygen sensor?
 

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Discussion Starter · #5 ·
New Update: Fiddled with some wires and it seems to have come back to and is no longer at 0.0

SHRTFT1 -15.6%
LONGFT1 6.1%
SRTFT2 -15.6%
LONGFT2 3.9%
MAP 41.0kPa
O2B2S1 0.640
SHRTFTB2S1 -16.4%

Why would it be running at -16.4% ? doesn't that mean it is pulling 16% fuel?
 

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Not what it means. 16% fuel would be super rich. 14.7 part air to 1 part fuel is the ideal. Called the stoichiometric ratio, but it is only ideal at a steady speed. Acceleration requires a little more fuel for a number of reasons. Most popular one is it burns cooler to prevent pinging.
14.7 to one is about 6.8% fuel. (1 ÷ 14.7)
SHRTFTB2S1 -16.4% - means the Short Term fuel trim as read at the the bank 2 (pass side) sensor 1 is lean 16.4% from perfect. The - sign is lean. No sign is rich.
Sounds to me like your O2's are working OK. All the numbers are with in normal ranges.
I'm sure a couple of the engineers that frequent the site could give you a longer, more thorough explanation or links to pages on the subject.
 

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Discussion Starter · #7 · (Edited)
Not what it means. 16% fuel would be super rich. 14.7 part air to 1 part fuel is the ideal. Called the stoichiometric ratio, but it is only ideal at a steady speed. Acceleration requires a little more fuel for a number of reasons. Most popular one is it burns cooler to prevent pinging.
14.7 to one is about 6.8% fuel. (1 ÷ 14.7)
SHRTFTB2S1 -16.4% - means the Short Term fuel trim as read at the the bank 2 (pass side) sensor 1 is lean 16.4% from perfect. The - sign is lean. No sign is rich.
Sounds to me like your O2's are working OK. All the numbers are with in normal ranges.
I'm sure a couple of the engineers that frequent the site could give you a longer, more thorough explanation or links to pages on the subject.
So just to clarify that is normal/safe ranges and I have nothing to worry about at this point?

Just to add these numbers are at idle.
 

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Discussion Starter · #8 ·
Alright so it threw the same codes again after taking it for a drive.

P0135
P0455

Live data shows the sensors fluctuating between 0.0 and -2.3 now and no longer -16 and -15 and B1S1 and B2S1 are showing voltage so I believe the sensors are working but maybe the heater circuit did fail on them both at the same time like the codes indicate.

They look like they been replaced before by the original owner probably with the cheapest available. Would these sensors still be showing data if the heater circuits in them fail or wouldn't they just be completely dead?

MAF is running at 5.1 if that helps any I don't know.
 

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Depending on the distance from the engine they may or may not work. IDR the temperature they have to reach to operate, but once they were moved from directly in the manifolds to a foot or 2 away GM did add the heaters.
That may have been simply to raise them to operating temp faster or maybe it's always needed. IDK.
The pink wire to each of the sensors is 12V from the O2 fuse. Make sure you have 12V at the connector with the key on. The ground wires ground back at the PCM so it can see that hey are working. The black on bank 1 and the light green on bank to are the grounds. Check the continuity from the O2 connector to pins 72 for bank 1 and 74 for bank 2. If you have 12V and ground to the sensors, they are bad. If you don't it's in the wiring somewhere. Look for abrasion or burns. Since they are relatively new I'd look at the wiring first.
 

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New Update: Fiddled with some wires and it seems to have come back to and is no longer at 0.0


SHRTFT1 -15.6%
LONGFT1 6.1%
SRTFT2 -15.6%
LONGFT2 3.9%
MAP 41.0kPa
O2B2S1 0.640
SHRTFTB2S1 -16.4%

Why would it be running at -16.4% ? doesn't that mean it is pulling 16% fuel?
Question 1: not certain, but do not care. Q2: essentially, yes, but only at the exact moment in time. LTFT for a bank is constantly updated by the ECM in an attempt to keep STFT around 0%. If a bank’s LTFT is stable in single digits on an old vehicle, especially low single digits, there is likely no major problem to be deduced by trims.



Not what it means. 16% fuel would be super rich. 14.7 part air to 1 part fuel is the ideal. Called the stoichiometric ratio, but it is only ideal at a steady speed. Acceleration requires a little more fuel for a number of reasons. Most popular one is it burns cooler to prevent pinging.
14.7 to one is about 6.8% fuel. (1 ÷ 14.7)
SHRTFTB2S1 -16.4% - means the Short Term fuel trim as read at the the bank 2 (pass side) sensor 1 is lean 16.4% from perfect. The - sign is lean. No sign is rich.
Sounds to me like your O2's are working OK. All the numbers are with in normal ranges.
I'm sure a couple of the engineers that frequent the site could give you a longer, more thorough explanation or links to pages on the subject.
Based on previous posts of yours, I know you are very competent in automotive technologies. However, you seem to not be presenting this in a manner that is consistent with my understanding. As I understand it, the ECM does not “read” trims, it “sets” trims in an attempt to achieve stoich; at least in non-aggressive driving scenarios. I do not know if the S-series programming has any sort of PE (power enrichment) mode for near or at WOT. Additionally, as I understand it, + or – trims do not determine tell us whether the engine is running lean or rich on that bank, only whether fuel is being added or pulled based on feedback the ECM receives. That feedback can be accurate or inaccurate.
 

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….Additionally, as I understand it, + or – trims do not determine tell us whether the engine is running lean or rich on that bank, only whether fuel is being added or pulled based on feedback the ECM receives. That feedback can be accurate or inaccurate.
Been thinking about my previous post. I think I also did not present this accurately. I think the following is a more accurate and helpful statement.

Additionally, as I understand it, + or – trims do not determine tell us whether the engine is running lean or rich on that bank, only whether the ECM is trying to add or pull fuel based on feedback the ECM receives. That feedback can be accurate or inaccurate.
 

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Discussion Starter · #12 ·
Alright so something I need to figure out. The sensors seem to be reading on live data and they have voltage according to live data. I will have to verify another time with a voltmeter.

But it again threw a code for the heater circuits on both sensors. But the sensors are working according to the live data. So is it possible that the heater circuits both failed at the same exact time? because it makes no sense that now the sensors are reading but the heater circuits are dead.

And is there anything that could have been hit when I had the bed off changing the fuel pump, any ground wire, connector, etc? it is a long bed regular cab. It is throwing the codes for bank 1 and bank 2 sensor 1 so the ones closest to the engine. It was a complete bed off job because I did rust protection on the frame.
 

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Alright so something I need to figure out. The sensors seem to be reading on live data and they have voltage according to live data. I will have to verify another time with a voltmeter.

But it again threw a code for the heater circuits on both sensors. But the sensors are working according to the live data. So is it possible that the heater circuits both failed at the same exact time? because it makes no sense that now the sensors are reading but the heater circuits are dead.

And is there anything that could have been hit when I had the bed off changing the fuel pump, any ground wire, connector, etc? it is a long bed regular cab. It is throwing the codes for bank 1 and bank 2 sensor 1 so the ones closest to the engine. It was a complete bed off job because I did rust protection on the frame.
How are you monitoring live data?

Heaters are generally only required to get the O2 sensors up to an “operating” temperature range. Once they are there, heat from the exhaust gases keeps them there IIRC. So, if the heater or heater circuit has failed, you can see a heater code and SES light concurrent w/ proper sensor operation after exhaust gases heat them.

  • LT GRN/WHT – Heater high (5V)
  • LT GRN – Heater low (GND)
  • PPL or PPL/WHT – Sensor high
  • TAN – Sensor low

All Heater high & lows are spliced together, respectively, at some point.
 

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Discussion Starter · #14 ·
How are you monitoring live data?

Heaters are generally only required to get the O2 sensors up to an “operating” temperature range. Once they are there, heat from the exhaust gases keeps them there IIRC. So, if the heater or heater circuit has failed, you can see a heater code and SES light concurrent w/ proper sensor operation after exhaust gases heat them.

  • LT GRN/WHT – Heater high (5V)
  • LT GRN – Heater low (GND)
  • PPL or PPL/WHT – Sensor high
  • TAN – Sensor low

All Heater high & lows are spliced together, respectively, at some point.
Scan tool is how I was monitoring it.
 

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Clarification, I think the heater circuits for Sensor 1's are spliced, but the Sensor 2 heater circuit is separate. Also, I do not know if the Sensor 1 heaters operate on 12V or 5V. So my wire color legend may be wrong with respect to that.

I do not understand theory of operation for O2 heater circuits very well. I do not know if Sensor 1 heater circuit is energized only for a while after startup or always during Key On Engine Running (KOER). I think the Sensor 2 Heater High is different with 12V being present (through the 20A “Oxygen Fuse”) when the key is in the Run or Start position. I do not believe that fuse has any relevance to the Sensor 1's. I think you will need to front probe (while disconnected) or back probe (while connected) the connector(s) to see what is going on with the heater circuit for one of the Sensor 1's. Both South Main Auto and ScannerDanner youtube channels probably have multiple videos showing O2 sensor heater circuit troubleshooting in great detail. If you want to properly troubleshoot, I suggest you search their videos as those can help you a ton. Also, I am sick of typing. Good luck. :)

ETA
I heavily edited the above post when I realized I continually used "Bank" in place of "Sensor". Sorry to anyone who read that mess and was confused because of how wrong it was.
 

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Thanks for presenting the info more correctly. As I said in post 6, there are folks better versed in exactly how the sensors and PCM do their job. I simple know which wires go where to make it happen.
As I understand your explanation, the computer does the work, The scanner simply tells you what it's doing. The computer does it's thing whether you're scanning it, or not and the scanner simply tells you what it sees happening.
 
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