PO155 2002 Chevy S10 4x4 4.3L 176,000 miles

Boomer

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Boomer

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Ok, I took a break from this for a bit. I am now ready to get back at it. I cleared the codes the other day. It only takes a drive or two to come back. I now see all kinds of codes. The codes repeated. I have a new mass air flow sensor and new 02 sensors. I've done the checks I've previously mentioned. I still experience no driveability problems. No increased fuel consumption. The truck thankfully runs fine. I just need a solid plan to attack it. Kinda like a troubleshooting tree.
 

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Boomer

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I've checked the wiring and the sensors are new. I even moved a sensor that was not throwing a code to a position that was and the codes did not change. I've seen the coolant temp sensor is another possible cause for this code. My scanner reports the same temperature as the gauge in the dash. At normal operating temp it's 196 degrees. That matches the dash gauge as well as the thermostat part itself. To me, that should rule out the CTS but, I'm just a weekend wrench turner not a pro mechanic. I've read that in rare cases, the PCM itself could be the cause. Is there a way to confirm or rule that out without part replacement?
 

billr

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It still sounds like a wiring problem to me (although that could be "wiring" right at or in the PCM); and we still need to have a schematic so we can try to guide you through more definitive trouble-shooting. If nobody else posts a schematic, you may have to purchase a manual/subscription so you can post it.

If you do want to throw a PCM at it , I suggest the $150 one linked-to below. It will come tested and programmed specifically to your VIN (or whatever other OEM stock config it may be in now). I have only ordered from them once; the processes for ordering and shipping tracking both seemed kind of flaky to me, I was suspicious of a scam. However, the unit arrived on-time and fit/functions perfect. No core cost/bother, either.

Oops, I forgot to give the link!

 
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grcauto

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The SES light came on. I checked it. It was PO155. Bank 2 Sensor 1 heater circuit malfunction. The heater wires on the sensor showed no continuity. I replaced it. Next up came PO138. Vehicle ran fine. I thought great, another sensor is bad. I cleared the code. It has not come back however, the PO155 comes back all the time now. I keep clearing it and it comes right back. I checked the new sensor and the heater circuit is ok. I checked the other upstream sensor. Heater wires have continuity. Ok, maybe it's the vehicle wires. I put my meter on the vehicle side of the harness on the terminals corresponding to the heater circuit. When I turn the key to the run position. I get system voltage. This happens on both upstream sensors. I could just replace the one I haven't replaced since I have a brand new Bosch for it on hand. I really hate being a parts changer and the old sensor has continuity just like a new one. Should I just replace it and see what happens?
Best to do a load test with a bulb that draws 4 amps or so. A high resistance short will not show without current flow.
 

Boomer

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It still sounds like a wiring problem to me (although that could be "wiring" right at or in the PCM); and we still need to have a schematic so we can try to guide you through more definitive trouble-shooting. If nobody else posts a schematic, you may have to purchase a manual/subscription so you can post it.

If you do want to throw a PCM at it , I suggest the $150 one linked-to below. It will come tested and programmed specifically to your VIN (or whatever other OEM stock config it may be in now). I have only ordered from them once; the processes for ordering and shipping tracking both seemed kind of flaky to me, I was suspicious of a scam. However, the unit arrived on-time and fit/functions perfect. No core cost/bother, either.

Oops, I forgot to give the link!

Hey Bill, thanks for your response. There are connectors close to the PCM (say about a foot or so away). They are the factory GM Weatherpack type. It was at this point that I checked the wiring through the sensor for continuity so, it does still leave the slight possibility that it's between the PCM and that point. I have a diagram from All data. It's not the easiest thing to see the pin numbers at the PCM itself. I'll have to make sure I find them and completely check it. A visual inspection has me highly doubting it but, I know PCM failure isn't that common. The wiring connections I could see at the PCM "looked" ok.
 

Boomer

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Best to do a load test with a bulb that draws 4 amps or so. A high resistance short will not show without current flow.
That is a great point. I should have thought of that. It seems to me that would also test the wiring all the way through from PCM to the load. I'm going to do that this weekend. Thank you GRC
 

billr

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If you have a schematic, post it! We can make a lot more sense of the symptoms if we know what the wiring connections are.
 

grcauto

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That is a great point. I should have thought of that. It seems to me that would also test the wiring all the way through from PCM to the load. I'm going to do that this weekend. Thank you GRC
These pesky little things are near impossible to find without putting the circuit under a load of at least 2 amps. I use an old 4 amp headlight. Works great.
 

Boomer

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These pesky little things are near impossible to find without putting the circuit under a load of at least 2 amps. I use an old 4 amp headlight. Works great.
What bulb works with alot of applications?
 

Boomer

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Ok, I checked the wiring today at S101. That's splice 101 in the diagram I posted. I know I checked at the sensors before and I had voltage. Today when checking, I had nothing on the LT.GRN/WHT wire coming from the PCM. I used the battery as my ground. I did however have system voltage on the pink wire in that harness. The pink wire goes to the sensor farthest to the rear. That sensor is NOT throwing a code so, that makes sense. It also makes sense that I get a code for both upstream sensors since they share the same power wire indicated by the diagram. I made sure I had continuity from the PCM connector wire down to that connector at S101. Very low resistance connections. This indicates to me that the PCM is not putting out the high signal voltage when the key is turned to the run position. I know PCM is the least likely cause of this code but, it sure looks like the culprit.
 

billr

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Agreed. it is likely that the O2 sensors are not powered up when engine is not running.

If you still get no voltage at S101 when the engine is running, go back to C2 pin 74 on the PCM itself. Open up the PCM and read voltage right at the pin that is soldered to the PCB. Also, will be a good opportunity to do a visual check for poor solder connections on the PCB...
 

Boomer

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Perhaps the PCM only powers up the O2 heaters when the engine is actually running?
I’ll have to check. I know the pink wire going to the rear sensor gets hot without the engine running. I guess the other two could be different.
 
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