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18 GMC same problem. Replaced MAF and MIT sensors. P026D persists but no CEL...yet. Says its a permanent code.
Also, bought new GM MAF sensor from other source and got a P0102 code. Paid big money and bought dealer version and code went away. FYI.
 

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Thanks for the clarification. I saw your other post on the topic and realized you were talking about the intake temp sensor. So, when you say permanent you mean, not pending, I get it.

I am not a truck/diesel expert, but this is what I am starting to think. There is nothing wrong with these trucks, that is why they cannot be fixed. The problem arises from the N192273630 DPF Regeneration recall that reprogrammed the ECM and TCM. This software update is flawed and causing the truck to throw codes when nothing is wrong. Maybe what we need is too go back to the old software 🧦
Actually my CEL issue started before the reflash. I eas
 

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Actually my CEL issue started before the reflash. I was hoping the reflash would solve it. Im curious on the tank issue / venting. Im planning to do a visual exam . With the factory emissions out at 50k im not real anxious to take it to the dealer. They knew what they were doing cutting it in half

[/QUOTE
 

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Got the same one on my 18. I thought it was related to the MAF sensor BUT I loosened my fuel cap and the code remains pending with no CEL. This comes from another thread about the tank vent being plugged. Another issue I have is the electonic brake controller canbus A offline / no com and canbus b offline / no com.
Im still digging. Wish i had my LML back.
 

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At the risk of beating a dead horse-
I've found that the P026D can relate to one of many potential problem causes. I've seen numerous posts regarding a filthy MAP sensor, where an outright replacement did nothing to eliminate this error. MAF sensor was also mentioned in several places and I even replaced mine- no permanent change. Another referenced an air intake problem where the airflow to the intake manifold was low (suggesting a loose connection somewhere between the air cleaner box and the intake or, a clogged filter). I've had both NOx sensors replaced (expensive "fix", w/o any problem resolution).

I noted that one day while refilling my DEF, I had slipped and somehow managed to make contact with the harness connector at the MAF sensor. I thought i saw a piece fall off, but couldn't confirm; however, from that point, I noted that the connector shell (harness side) was sitting at roughly a 30 deg angle to the sensor- not "in line" with it. The wiring harness was "tight" IMO. I removed the harness mounting tabs (at the air cleaner box) and pulled up on the harness, relieving the strain at the connector, then ty-wrapped it into place at other points. After clearing the codes, I had in excess of 260 "warm-ups w/o an emissions malfunction" (WwoEM is what I think shows on my Edge CTS monitor). A good jarring from a pothole will rattle the harness enough to create a momentary break in the MAF sensor's connection and the code returns. I'll clear the code and we continue on.

Bottom line is that my problems may still be hidden and given the complexity of these emissions systems, it may never fully be resolved. If there was a "hard failure" of a sensor, I'd expect this DTC to consistently repeat itself. But it doesn't. To eliminate one variable, I'm considering taking the truck to the dealer and having them replace the connector shell at the MAF sensor as it still appears "very loose". As an aside, just remember that when you clear the error codes you also reset all of your emissions ready flags- which can take several hundred miles to reset on their own. This will be important if you are up against an emissions check as required by your state's DOT.
A reset is not a biggie in my world. Truck won't regen with CEL on and my scanner can force a regen if needed.
I'm personally tired of fighting city hall myself. This past weekend I went chasing after the fuel rail pressure sensor connector ( TSB for P026D ) which requires removal of the cold CAC line and I found oil in the CAC. The cold boot was wet with oil and the temp sensor had oil on it. This is not a good thing especially at 80k.

Im in touch with the dealer and if he thumbs down the oil in the CAC this pig will get a full coat of lipstick and we will see her later.

JMO
 

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Yes and no. When I pulled CAC cool side I found oil so I stopped there. This meant there was oil all through the cooler and at the time i did not know this was normal. I wanted to preserve the evidence for a dealer examination.

Right now I'm in the que for a dealer appointment. Later research tells me this is normal however and likely the dealer will say same and boot me out under the 5/100 drivetrain warranty.
 

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I've got the same bane of a code on my 2018 GMC L5P with it starting this spring at around 60K miles. The dealership flashed the computer, loaded a new program and sent me on my unhappy way. It returned within a week so I brought it back in and they replaced the MAF sensor. That lasted about 3 weeks.....I had the code deleted this week and are waiting it to return now, especially after reading your posts.
My question at this point is it worth hanging onto to this truck and what harm will be done running with that code?

I bought an extended warranty with it and there doesn't seem to be a real clear answer as to a fix. The dealership seems to be clueless and because I've changed my fuel filters in the past (earlier than regularly scheduled) they've hinted that warranty will not be covered.
I'm disgusted and at a loss as to what to do now.
Im with ya bud, 18 GMC with constant CEL and now the dreaded P289 code which is DEF quality, or a NOX sensor or some other unknown. Ive read where dealers throw a NOX sensor at it and nah, thats not it so...they don't know. I threw in 5 gals of Blue Def thinking maybe it would help out. Had an issue like this at 10k miles. I left the truck at the airport and it was 70 degrees, I came back to 43 and rain. 10 miles from the airport the dash lit up like a Christmas tree and DIC went into bitch mode. I stopped and threw in 2.5 gals of DEF and DIC shut up but still had a CEL. Cleared that and rest of the trip was good.

I'm at 80k with no warranty other than 5/100 and a CAC with oil in it plus the CEL. In the South there is a two week back log for a GM diesel tech AND CHEVROLET CANT WORK ON MY GMC UNDER WARRANTY. Thank you GM. All being said I think its time for her to go. Prices are outrageous and I'm still under 100k. Carvana offered 45k and I think I can get 50 odd on the street.

I share your disappointment.....
 

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Why can’t Chevy work on your GMC? I’ve had warranty repairs done at the local Chevrolet dealer on my GMC. Just curious?
According to the service writer at the Chevy house he is not allowed to perform warranty work on a GMC. He says folks were getting tired of poor service at GMC so they flocked to Chevy, this pissed off GMC dealers so they complained. GM made this policy.

He says he can do CP and recall work / safety work but not things like injectors, NOX sensors etc under a factory warranty. Extended, yes, outside type yes and of course CP.

That's what he told me ...in the service drive when I showed for my appointment.......knowing all well I had a GMC.
 
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