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I've got a 2015 GMC Canyon with the 3.6L, Auto, and 4WD. It sets a P2138 (Accelerator pedal position sensor correlation) code intermittently. Back in December it set the code and the guy had Autozone read and reset it. The code came back within a day or two. He brought it to me but I was unable to duplicate it. Everything worked fine. APP Sensors 1 and 2 were perfect with sensor 2 being 50% of sensor 1. It was perfect both on the scan tool and on the scope. Because it's a work truck, and it's not very expensive, he opted to just replace the pedal assembly to hopefully not have it come back.
That was over 4 months ago without a problem. Now it set the code again on him (it goes into limp mode when this happens.) Today I spent hours on it trying to duplicate the problem but after a few key cycles the MIL went off on its own and I cannot find a problem anywhere.
History of the truck is that this guy bought it in 2015 with 10K miles on it and it has about 60K on it now. During my troubleshooting today I found that the connector to the APP sensor has been replaced (butt splices) on all the wires, and a bypass wire was run from the pedal to the PCM for the signal wire on the sensor 1. I didn't even notice at first because they used the factory tape on the wires. It's obvious I'm working with a truck that the dealer couldn't fix under warranty and they tried a new connector and a new wire. My guess is the original owner traded it in or it was a lemon return.
In my freeze frame data (under OBDII) it shows Acellerator Pedal Pos D at 23.9% and Acc Pedal Pos E at 9% so it's obvious that the signals were not correlated at the time it set the code. Since live data shows 9.8% is the resting position of the Accelerator pedal Pos E that tells me that sensor E (or Sensor 2) is the one with the glitch since 9.0% is below the resting position. In all my troubleshooting I was unable to duplicate 9% at any time.
There is a TSB on this for all GM vehicles from 2005 to 2015 which discusses moisture intrusion, possibly in the kickpanel at a connector but this truck has no connector in the kickpanel for this. There is the APP sensor connector, a 45 pin connector under the hood, and the PCM connection and that's it. I inspected the 45 pin connector and couldn't find any problems there. I inspected the splice at the PCM for the bypass wire and it seemed good (the splice is for sensor 1 or D) so I'm not suspecting that. I have wiggle tested all the spliced wires so while it looks like a hack job it seems to have been spliced properly with weather proof butt connectors. I did not unplug the PCM and ohm check all the wires, as I don't want to take the chance of damaging the PCM and I'm not sure what that would prove since everything works right now.
Has anyone seen this on such a new vehicle? I suspect possibly a bad PCM but I would think the dealer would have changed that out before hacking up the wiring harness--they love changing PCM's and programming. It's not something I can do.
With this being so intermittent I'm about at my wit's end with it. Thanks for any suggestions anyone has...
Ninjaxchicken wrote: Dual throt pos sensors? God jesus why
It's redundancy for the electronic throttle, for safety. It has 2 pedal position sensors at the pedal and 2 throttle position sensors in the throttle body. That way if 1 sensor malfunctions the PCM knows it's a malfunction and not a correct signal.
It's not that complicated, really. It's two potentiometers in the pedal and two three-wire circuits to the computer with connectors at the pedal, a big 45-pin under the hood, and the PCM connectors. It's sad that the dealer doing the warranty work (I'm assuming since it happened prior to 10K miles) had to hack up the wiring but I can't find any problem with the splices or bypass wire they ran.
I wish I knew if they already replaced the ECM or not but I've been over the wires and can't find a problem which leaves either several bad pedals or a bad ECM.
I was thinking that a PCM ground issue could be a cause so I checked that and cleaned both of the battery cable body grounds from to improve connections there. Since the problem is intermittent I'll never know if I improved anything or not until I give it back and hope it doesn't set the code again.
I did notice that the replacement pedal the guy put in (he bought direct from the local dealer) is dated 10/2015 so it's been sitting around a while but that's not necessarily out of the ordinary either.
Hey that's a real head banger for sure ! I would definitely disconnect pcm and pedal sensor and check resistance, short to power, short to ground, and lastly load test all the wires to app (you'll have to put power or ground to 1 end and a light and volt meter at the other) I suspicion you'll find your problem with the load test .
Yeah, I wanted to load test but I'm just leery of disconnecting the PCM. I'm just a guy in my garage doing work for others on the side and the last thing I need to do is damage a PCM because I disconnected improperly or something. Alldata says you need to disconnect the battery first but then I lose all my memory and freeze frame data. Not the end of the world but my fear is plugging it back in and having it not start or some stupid thing. If this truck was a little older I probably wouldn't be so concerned.
I see what type of scan tool do you have ? Would you be comfortable changing a battery in this vehicle? Worst case scenario some limits need relearned, once battery is disconnected modules can be disconnected and reconnected without any problems (as long as you don't do your tests with the module plugged in it will be fine)
Update:
The truck threw the P2138 code and went to limp mode again last night. I went to where the truck was with my scan tool and the code was there. The live data showed sensor 1 at 19.6% and sensor 2 at 7.5% which is not 50% so the problem was happening. Unfortunately, it was in the Lowe's parking lot and I didn't have my scope. The guy left it there and dropped the keys off to me this morningand I went there with my scope this morning, first thing. Unfortunately, when I turned it on the problem was gone. Sensor 1 showed 19.6% and sensor 2 showed 9.8%. This told me the problem is in sensor 2 (the 50% sensor.) Nothing I did would make it fail, though. I wiggled every wire at every connection. I moved the pedal multiple times. I tapped on the ECM. The truck has been fine since April so it sets the code once every 2-4 months!
I brought it back to the shop and did the load testing on the wiring as suggested above. I load tested sensor 2 wiring first from the ECM connector to the pedal connector. Each wire lit a halogen headlight bulb with no problem--I didn't measure the amp draw but it has to be more than enough for that signal circuit. I also checked sensor 1 wiring and it was good too, but not suspect to begin with.
So, since the pedal was replaced, the wiring is good, that just leaves the PCM. The owner called the GM dealership where the truck came from at 10K miles. They wouldn't give him any info on the work done and gave him a number to GM headquarters somewhere. They wouldn't tell him anything either but he asked if they could tell him if the ECM had been replaced and they told him it had not.
Unless we've got another faulty pedal I suspect a bad ECM but it's tough to make the call. RockAuto has a new AC Delco ECM for $170 and I assume a program will cost around $150 or so. It's not a huge amount but I sure hate to make the call if there's something I'm missing. I sure would have liked to check the 5v references to both sensors when it was failed but missed the chance. So frustrating.
I work for a big car rental company. We have several 2017 to 2018 Malibus with 2.5 code P2038 GM does not have a fix. I have tried similar diagnostic can not catch it. If I clear code ok only 5 percent come back. They replace pedal if it comes back replace harness from pedal to PCM. As far as I can tell no fix or GM not giving info out. I figure it is going to be a recall soon. Limp mode not safe for drivers. I will follow up if I here any thing new.
carlosdz61 wrote: I work for a big car rental company. We have several 2017 to 2018 Malibus with 2.5 code P2038 GM does not have a fix. I have tried similar diagnostic can not catch it. If I clear code ok only 5 percent come back. They replace pedal if it comes back replace harness from pedal to PCM. As far as I can tell no fix or GM not giving info out. I figure it is going to be a recall soon. Limp mode not safe for drivers. I will follow up if I here any thing new.
Thanks. Did you mean P2138?
I replaced the connector again because it was only $22 on Amazon for the GM replacement and the hack job done by the dealer didn't look too good, though I was never able to create the problem by tugging on any of the wires. There was also a set of aftermarket back up sensors that were hooked up to the trailer connector and had a wireless transmitter. When he bought the truck it didn't have the head unit for the sensors so I pulled that whole box out. There is a service bulletin that discusses aftermarket equipment connected to the 5v or the low reference causing P2138 codes. I don't think it was the issue and it wasn't connected to either but better to remove it as a variable.
So, truck is out again waiting for the problem to occur again. Assuming the replacement pedal we installed is good, the only thing left to replace is the PCM.
Yes P2138 one of our guys says he found 5 volt ref wire rubbed thru at heater hose clamp but I have not found that. I thing because it is the same code on the same vehicle it has to be PCM or software update? The same wire can not rub at the same point on so many cars. Also same circuit wire causing same code like high res same wire? The Malibu uses 2 separate 5V ref for both APP sensors. Each have 5V ground and signal 6 wires total from APP pedal to ECM. It still can be bad APP pedals like a bad batch from manufacture. GM has to eventually answer this problem. I just looked up your Canyon has identical circuits.
That is exactly how the 2015 Canyon is wired. The harness goes through a 50+ pin connector up near the battery and then from there to the PCM. The dealer ran a new wire for the #1 signal from the pedal to the PCM, bypassing that connector, but I've found my issue to be on the #2 circuit.
Do I tell the guy to get another pedal or go ahead and swap out the PCM? I'm at a loss as to what to do next. I can't do programming so don't have that option. He'd have to take it to another shop, or the dealer, for that.
The cars i work on are still under warranty so my company gets reimburse for parts and labor. I did send one to the chev dealer they clear code and drove 40 miles code does not come back. Your car is older so you might want scope the APP wires at PCM and see if you see drop out? i have access to repair information so if you need schematic info let me know.
carlosdz61 wrote: The cars i work on are still under warranty so my company gets reimburse for parts and labor. I did send one to the chev dealer they clear code and drove 40 miles code does not come back. Your car is older so you might want scope the APP wires at PCM and see if you see drop out? i have access to repair information so if you need schematic info let me know.
If you read the thread from the beginning you can see I've done all that. I first got the truck back in November/December. It had set the code twice in two days. By the time it got to my garage it was fine. I scoped both channels and it was all perfect. channel 2 was 50% of channel 1 and no dropouts. The guy decided to just replace the pedal. After replacing the pedal it was fine for nearly 5 months when the code set again. I went through everything, short of disconnecting the PCM and load checking the wires. I cleaned up a body ground and a frame ground and checked the computer ground on the block. It was fine for nearly two months and then set the code again in the Lowe's parking lot as described just a few posts up. I went straight there but had only my scan tool. I could see the problem happening. At idle and key on engine off, channel 1 was at 19.6% and channel 2 was at 7.5%. That is enough to set the code. The next morning I went to get the truck and the problem was gone. Channel 1 was at 19.6% and channel 2 was at 9.8% and nothing I did would duplicate it. The freeze frame data for the code indicated the same difference in channel 2 so I'm certain it's a problem in channel 2 on mine. I then disconnected the PCM and the pedal and load checked all the wires and they all light a 5A test light with no problem.
This guy bought the truck with 10K miles on it and has had no work done to it since he bought it. I found the connector had been replaced and a bypass wire run from the pedal to the PCM for channel 1 signal wire. That told me that I had inherited a problem the dealer couldn't fix.
So, now I'm down to either putting another pedal in it or replacing the PCM. The date code on the replacement pedal was from the same time period so maybe a batch of bad pedals? The dealer won't tell us anything they did except that the PCM has not been replaced.
It sure would be nice to know what's happening on the 5v line for sensor 2 during the fault to kind of narrow it down between the APP sensor and the 5v regulator in the PCM.
The dealer apparently bypassed the #1 feed, you could try bypassing the #2 feed for a toss of the dice while you wait for it to happen again.
Noah wrote: It sure would be nice to know what's happening on the 5v line for sensor 2 during the fault to kind of narrow it down between the APP sensor and the 5v regulator in the PCM.
The dealer apparently bypassed the #1 feed, you could try bypassing the #2 feed for a toss of the dice while you wait for it to happen again.
Sorry, I missed this. I don't recall getting an email for your reply.
Yeah, when it was hard failed in the Lowe's parking lot my plan was to scope it and check the 5v ref on both circuits. Unfortunately, by the time I got my scope to it the problem was gone. It's been extremely frustrating since I'm never able to duplicate the problem. If I could duplicate it I'm positive I could positively diagnose it. Right now it's been fine since the Lowe's parking lot event.
I really suspect the ECM at this point. Either a slight drop in the 5v ref or signal wire internal to the ECM is what I think it is but I just can't confirm anything. A Delco ECM is only $170 on RockAuto but obviously has to be programmed and I don't have that capability.
Correct me if I'm wrong here but t seems to me if it was the pedal then when I moved the pedal it would likely fix itself. When I had it failed in the parking lot it kept that error in the scan data throughout the range of the pedal movement, indicating either a ground issue or 5v reference issue on the #2 circuit. Does that sound reasonable to assume?
The following user(s) said Thank You: aboudi.naddaf
The truck hasn't thrown the code since mid June. I replaced the APP connector that the dealer had replaced, and removed the aftermarket backup sensors that weren't hooked up to a head unit but were tapped into the reverse light circuit.
I've come up with another theory based on a video posted today by Eric O. at South Main Auto. It was on an older TrailBlazer with a P2138 that turned out to be a bad radiator fan pulling down the 5v reference on one of the APP circuits. It got me thinking...maybe something (other than the PCM) is pulling down the 5v reference. On this truck there are 4 separate 5v reference circuits and they're labeled 5v1-5v4. Since I'm pretty suret the APP sensor 2(E) is causing the issue I looked at the diagram again. That APP2 circuit is connected to 5V3 (with APP1 on 5V4.) According to the 5V ref diagram there is only one other item on that 5V3 reference circuit. That is the MAP sensor. I've never seen a MAP circuit code on this truck but I would think it's conceivable that the MAP sensor could intermittently pull the 5V3 reference circuit down, even slightly, and that could be enough to throw the APP 2 circuit off and throw the P2138 correlation code without throwing a MAP sensor circuit code.
I think if this truck throws the P2138 code again (assuming it doesn't throw the code long enough to confirm a 5V reference circuit issue) that the MAP sensor might be a better part to throw at it as opposed to replacing the pedal again or the PCM. Freeze frame data with a specific look at the MAP sensor reading might also be in order.
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