Code P2138, Reduced Engine Power and Water Intrusion

[ARAMP1]

I have approximately 200K miles on my 2011 Yukon XL 2500 (6.0L/6L90E). As the title says, I'm getting a P2138 code with reduced engine power. I've seen a couple threads on this, most recently THIS ONE.

For troubleshooting, I followed THIS video. Resistance was over 5Ω in both low reference wires and I found one of the wires frayed so I went ahead and replaced all six wires to the throttle pedal (new wires in the harness, new plug and repined with new Delphi connector). Now both low reference wires (purple and brown) are exactly 2.2Ω (under 5Ω) and both the 5V reference wires (tan and white/black) are exactly 5.1V (over 4.8V). I replaced the throttle pedal for good measure.

One odd thing, my brown and white/black wires were swapped around from this video. I went ahead and swapped them around to match what was in the video to see if that was the fix but it didn't change a thing. Since I had replaced all the wires and plug, I still had the old one. Then, I found this diagram online which is what mine is...so I didn't accidently swap those wires.



I had a lot of the plastic wire loom under the hood brittle and missing so I decided to replace the six wires and connector from the ECM to the throttle body. The throttle body was gunked up so I replaced it to included a new TPS.

I've seen the TSB related to water intrusion and the P2138 code. I do have water intrusion on the driver side. I've cleaned out all of my sunroof drains and have drained water in them verifying they are clear. I just recently pulled the GPS antenna and put some RTV around the opening though it hasn't rained since doing this. I remember reading a thread here where someone had a leak around a windshield/body seam. I haven't explored that yet.


So now I'm kind of at a loss. Both throttle position signals are still off from each other.




I've checked all circuit breakers and all relays appear to be fine. At this point, I guess I'm just throwing parts at it to figure out what will fix the reduced engine power and P2138 code. I'm trying to figure out what module under the dash may have gotten corroded due to water intrusion. Maybe a bad ECM (the vehicle starts and idles fine)? Appreciate any ideas or suggestions for troubleshooting.

 

[mikez71]

Might check the TPS for correct installation.. Needs to be installed with throttle blade open.

 

[ARAMP1]

Might check the TPS for correct installation.. Needs to be installed with throttle blade open.

I installed a full throttle body that included the TPS. I assumed it was all good, I suppose it could have been installed incorrectly at AC Delco. Although when I did the throttle body relearn procedure, you could hear the throttle valve opening and closing.


I'm beginning to thing that water intrusion got to the body harness connector on the firewall under the dash. Though I've pulled all connectors and there are no signs of corrosion, that may have caused a short in the PCM. I'm not sure what else is left.

 

[mikez71]

Maybe check the pedal position to make sure it responds smoothly to your pedal inputs?
Just to rule that out..

I would think ACDelco installed it properly, yet I wouldn't be totally shocked if it was off for some reason..

 

[PLohr]

Isn't P2138 having to do with the accelerator pedal position sensor (APP / APPS)? This is the description from the scanner for P2138: "APP sensor 1-2 correlation".

Why do you think the TPS is the trouble based on the P2138 code? Am I misunderstanding something?

Here is P2138 from Alldata:

Thought I'd share this in case anyone else has a similar issue. I am working through a P2138 diagnosis on a 2008 Yukon XL 6.2L. No signs of water intrusion as the TSB suggests (not ruling it out though). Resistance through the low reference circuits internal to the ECM is higher than allowed. Four tests have shown those two circuits well above the 5-ohm maximum. Those four tests showed between 15 and 50 ohms (too high, and a wide range too) on both circuits. The two low reference wires (ECM and APPS unplugged) show 0.3 ohms resistance (not the trouble). I also tested the two low reference wires for short to ground - both were fine. Seems like a bad ECM? I'm trying to figure that out. Then also figure out if there's a condition, such as water intrusion, that caused it.

Thanks for listening to this rambling.

 

[OBSandaNNBS]

I installed a full throttle body that included the TPS. I assumed it was all good, I suppose it could have been installed incorrectly at AC Delco. Although when I did the throttle body relearn procedure, you could hear the throttle valve opening and closing.


I'm beginning to thing that water intrusion got to the body harness connector on the firewall under the dash. Though I've pulled all connectors and there are no signs of corrosion, that may have caused a short in the PCM. I'm not sure what else is left.

Hey @ARAMP1 how did this shake out?
I had the notorious "throttle body failure" but long story short, after replacing everything, it was a broken wire. I also inadvertently "fried" my brand new pedal sensor because of the wiring. So the stealership said.
Just something to be mindful of, if you don't correct the actual problem, you can damage your new parts. And you never suspect them, because they are "new"

 

[ARAMP1]

Isn't P2138 having to do with the accelerator pedal position sensor (APP / APPS)? This is the description from the scanner for P2138: "APP sensor 1-2 correlation".

Why do you think the TPS is the trouble based on the P2138 code? Am I misunderstanding something?

Here is P2138 from Alldata:
View attachment 460876


Thought I'd share this in case anyone else has a similar issue. I am working through a P2138 diagnosis on a 2008 Yukon XL 6.2L. No signs of water intrusion as the TSB suggests (not ruling it out though). Resistance through the low reference circuits internal to the ECM is higher than allowed. Four tests have shown those two circuits well above the 5-ohm maximum. Those four tests showed between 15 and 50 ohms (too high, and a wide range too) on both circuits. The two low reference wires (ECM and APPS unplugged) show 0.3 ohms resistance (not the trouble). I also tested the two low reference wires for short to ground - both were fine. Seems like a bad ECM? I'm trying to figure that out. Then also figure out if there's a condition, such as water intrusion, that caused it.

Thanks for listening to this rambling.

Essentially, it's unrelated...I think.

About a year prior, it threw a code that lead me to replace the TPS. When this P2138 problem arose and I found frayed wires going to the pedal, I checked everything else and found some frayed wires under the hood to the TPS (and a couple others). After replacing the wires, I cleaned the throttle body out a bunch but couldn't get it totally clean, so I just went ahead and replaced it for good measure. I thought the frayed wires to the TPS could have been a problem but ultimately the problem still exists after replacing them.

Hey @ARAMP1 how did this shake out?
I had the notorious "throttle body failure" but long story short, after replacing everything, it was a broken wire. I also inadvertently "fried" my brand new pedal sensor because of the wiring. So the stealership said.
Just something to be mindful of, if you don't correct the actual problem, you can damage your new parts. And you never suspect them, because they are "new"

Since it's not a daily driver, it's been on the back burner...still with the problem. I'm going to have a few days off work next week to dig back into it. Right now, I need to replace the battery. My plan is to go back and run measurements on the wires and the pedal. I have a new ECM in hand ready to go in.

Really just want to yank everything and drop a Duramax in.

 

[PLohr]

Understood.

Interested in how this works out, @ARAMP1 .

 

[PLohr]

Isn't P2138 having to do with the accelerator pedal position sensor (APP / APPS)? This is the description from the scanner for P2138: "APP sensor 1-2 correlation".

Why do you think the TPS is the trouble based on the P2138 code? Am I misunderstanding something?

Here is P2138 from Alldata:
View attachment 460876


Thought I'd share this in case anyone else has a similar issue. I am working through a P2138 diagnosis on a 2008 Yukon XL 6.2L. No signs of water intrusion as the TSB suggests (not ruling it out though). Resistance through the low reference circuits internal to the ECM is higher than allowed. Four tests have shown those two circuits well above the 5-ohm maximum. Those four tests showed between 15 and 50 ohms (too high, and a wide range too) on both circuits. The two low reference wires (ECM and APPS unplugged) show 0.3 ohms resistance (not the trouble). I also tested the two low reference wires for short to ground - both were fine. Seems like a bad ECM? I'm trying to figure that out. Then also figure out if there's a condition, such as water intrusion, that caused it.

Thanks for listening to this rambling.

Important update to my post. I did not know about the procedure for measuring "low reference" circuit resistance. I followed that procedure, still at 15 ohms of resistance (too high). I noticed the navigation disc is being spun by the radio / navigation unit endlessly. I don't know if that's normal. Pulled the "RDO" fuse (#42, underhood), resistance dropped to 2.1 ohms (well under the 5 ohm maximum). The ECM is probably ok.

I allowed myself to be fooled by a faulty new Dorman APPS (699-105). No resistance change when the pedal is moved. Called Dorman to verify it is faulty, they said "yeah, send it back to us." A new GM APPS is on the way. I like Dorman because they have decent customer service and technical support.