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2021 Denali HD
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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.....
Why can’t Chevy work on your GMC? I’ve had warranty repairs done at the local Chevrolet dealer on my GMC. Just curious?
 

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2017 Denali HD
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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?
Me too. Just had my NOX sensor and DEF system repaired under the 100k power train and 120k emissions warranty at the local Chevrolet dealer.
 

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I have been battling the same code now for a few months. Happened when we were at a rest area halfway up GA. Got it reset, came back on 600 miles later. Reset it again, came back 20 miles later. Took it to a diesel guy, started off with a plugged up air filter--reset values on it and it went away for about 700 miles until it came back on the other day.

Dropped it off at dealer and a few hours later they are replacing all 8 injectors. They said 1 was faulty, but better off replacing them all at once.

(Under warranty, 2017 with 53k miles)
Please let us know if the new injectors took care of the issue. If yes I'm going to have a talk with my dealship regarding this. I've got an extended warranty (Thank God) but the worst part is being without "MY" truck which is my mobile work platform/office.
 

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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.....
I've not had the P289 yet.......but did have a trailer brake code when I was not pulling a trailer? I had the dealership check (and charged me) that out last time in for P026D and they blamed it on my trailer.LOL!!
As far as DEF I've always used "Blue" and then switched to O'Reily brand due to cost but have not had any issues, knock on wood.
Having an extended warranty on this truck was well worth it so far. My prior truck was a 2014 Denali HD with an LML engine that blew a head gasket at 101 k miles, I've nearly lost all confidence in GM and these engines.
Thank's for all the info, it's been a great deal of help since the dealership remains to be clueless.
 

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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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For those folks who may have the check engine light come on and following code comes up: P026D I want to let you know that for me it was the MAP sensor being dirty. It has happened twice in the last few months and I have been able to clear the code by cleaning the sensor. I looked up the code on line and could not really find a solution or clear explanation. I am not sure what the reason is for the sensor getting dirty so soon again after the first cleaning which was about 3 months ago but I just wanted to post this to try to help out. My truck has 64K miles and I am the only owner so far. Anyway hope this helps.
Did that didn't work Replace air flow sensor also replace all injection and fuel line
 

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2017 GMC Denali 2500HD, 3.5" Rough Country Lift, 305/55R20 Yokohama Geolandar A/T
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I bought my ‘18 with about 99K on her. She lit up with P026D just after 100K. No warranty. Replaced MAP, MAF sensors, cleaned and seated fuel rail pressure sensor connector, checked the fuel tank, replaced fuel filter, air filter, even checked the injector rates. Nothing fixed it. Was about to take EGR off and clean out soot. Decided to just run it with the light for a camping trip. Pulled my 16,000 lbs 5th wheel for over 1,500 miles and the light went out. She just wanted to be rode hard and be beat. I still think it’s an issue with soot building up but letting her get a good burn seemed to help. Glad I didn’t replace all my injectors like many had advised. So if you can put your truck to work I’d say try it. Or clean your EGR and cooler. I’ll probably get around to doing that.
 

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My P026D code did not show up until around 60K and I have not changed my driving style. As the sensors age do they become more sensitive?
MAF sensor was replaced towards the end of June just prior to a camping trip and the code returned after 2 weeks, just after we returned from camping, my trailer loaded is approx. 8,500 lbs. On a normal day I have just shy of 2,000 lbs of ladders and tool boxes on it. I also drive an average of 600-700 miles per week both on and off the express way.
Since the code returned my CEL has cycled on/off a few different times with all codes being stored and all are P026D. A friend of mine has a 2018 High Country with 80K on it and he's never had that code and works his truck less than I do.
I understand your theory about working it and have always thought I have.
My truck was just dropped off at the dealership this morning for the 3rd or 4th time w/ P026D code, I'll let you know what (if anything) they find this time.
 
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