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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 Cooper Rugged Trek, Paragon Bed Cover
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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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Hey sumyunguyz, please post up again if this does or does not solve your P026D. Also, if your repair ticket states how the service tech identified the bad injector, would you post that too? Was is simply a balance rate out of spec or something more involved?

Thanks
 

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OK, so my code came back after the fuel flush, filter change, in-tank pump replacement. I dug into this code to find out that the code is triggered by low oxygen (O2) readings by NOX sensor 1 (under the hood, driver's side, just in front of instrument cluster). The NOX sensor measures both O2 and NOX. I started to monitor the NOX-1 sensor on my Banks iDash and saw that that is was taking a long time to turn on and then the readings were highly variable. The dealership tested it and found it was taking up to 20 minutes to turn on. They said it has an internal heater that was "open" and not turning on. It was replaced under the emissions warranty. This was the first thing they replaced when this problem started a year ago. It seems the replacement NOX-1 was faulty from the start.

If you have this code and a Banks iDash or somethig similar, set it up to show NOX-1 output. Per GM spec, the sensor should give output within a minute or two of start up. Also, it seems the readings were all over the place before, but now they are much more stable. I will monitor it on my next drive and report back.

Fingers-crossed this is the real solution-
 

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After my last posts, I took the truck out in the country and opened it up 5 or 6 times. The CEL went away on its own and stayed away for a week. But then it came back. Chevy did 18-NA-361 under warranty, you can find it online. It relates to particulates in the fuel filter housing. They clean the housing, replace the filter and flush the fuel supply lines ( supposed to be 2-3 gallons). This one is a little scary to me. The document mentions metal in the housing from manufacture or a clogged fuel tank vent as the cause of the particulates.

Chevy said if the particulates got past the filter into the injectors or engine the codes would have been P0171 (fuel lean), P0172 (fuel rich) or the dreaded P0300 (engine misfire).

Anyways, I think they are on to something. I hooked my code reader up on the drive home. I got P026D pending (no CEL) almost immediately and cleared it. P026D pending (no CEL) came back almost immediately, I cleared it. P026D pending (no CEL) came back almost immediately, I did not clear it and drove home, about 7 miles all together. The point is, it used to take many miles before P026D pending showed, so whatever they did, maybe they did not do it enough, but it seems to have triggered the condition. I think I read somewhere it takes 17 consecutive seconds of P026D pending to get a P026D and CEL.

The struggle is real and the saga continues. I really like this truck, but part of me thinks I would have been better of keeping my 98 Dodge.
I have the same problem. I took the truck to the dealer and they kept it for three weeks. This last time. Had this issue for 2 months at 83000 miles. First I had a ignition quality too high code. They cleared it. Changes the fuel filter and sent me on my way. Then it came back almost 200 miles later. So took it back in. Re programmed the pcm told me they drove it for 160 miles no light come get it. Upon pick up light came on when I turned the truck on. Same code. Told me to drive it to see if it would go away. I drove it the rest of the day Friday then Saturday morning no light. Drove all day Saturday 130-150 miles. No light. Woke up Sunday morning to go to work and there was the little yellow light. I was livid. So I took it back to the dealer and then they ordered all the parts to do a complete fuel system change. From the injectors all the way back into the tank. I’m capped my whole truck to do it. So I came to pick it up with no light on thank god. Drove it for two days. Sunday mid day the light comes back on. Ready to drive this thing into the ditch. Jk. But only this time it’s the p026d code now. Which stated the injector quality too high. So it says replace Nitrogen Oxide sensor. So with all that has anyone changed this sensor and did it resolve the issue
 

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I think NOX-1 sensor is the root of P026D in my truck. They replaced it and the truck drove better and got better mileage. But one week later the new NOX-1 sensor burned out again. It is taking ~5 minutes to turn on and should only take 1-2 minutes per GM spec.

My truck is going back in tomorrow morning. I just wonder if my dealer got a bad batch of NOX-1 sensors or if something is wrong with my truck making it burn out the heater on the sensors.
 

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I have a 2018 Denali HD, had the P026d code come up with the check engine light, its been at the dealership for the past 3 weeks waiting on a fuel tank because they are saying it collapsed. From everything I've been reading I don't see where this is going to cause or fix my problem. Anyone else had any issues with the fuel tank collapsing?
Yes! In my 2015 high country 2500. Died going down the highway "twice" and had it towed in. Showed 1/2 tank of diesel. Dealer said it showed empty when dropped off. One gallon got it running. I picked it up and it did it again a week later, same scenario. They said I once again I ran out of fuel. I called BS to that on the phone and told them check fuel Guage because it showed me 1/2 tank. They finally told me tank had collapsed due to faulty vent.
New tank, filler line, internals, and cap. They showed me the tank and it was sucked in pretty bad. All warranty (extended) or factory defects. Anyway, no charges for all work and parts.
 

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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.
Same here. i have learned how to clean the MAP sensor and it has done wonders. I was shocked the first time i removed the MAF; there was so much soot on that little sensor it could not have been reading anything. I also believe keeping this clean has increased my fuel mileage.
 
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