Received message today 171 miles until speed reduced to 65mph with 5,395 miles on my 2021 GMC Sierra Denali 3.0 Diesel.

Lmillsap said:
Sorry, don’t remember the brand (truck came with them).

Update:

Headed home with running boards disconnected. Pulling TT, 55 mph, 7th gear, 1800 rpm, cruise control, tow-haul mode, slight grade on interstate, registering about 14 mpg (truck isn’t working too hard). Slight drop in power and mileage (12 mpg) for 10-15 seconds, than trans slams into 6th (as indicated by 300 rpm increase), then immediately shifts smoothly back into 7th (300 rpm decrease). Stopped and checked OBD codes. Only one (P0336 Crankshaft Position Sensor Performance). Continued on towards home. Same thing happened two more times. It then happend a fourth time with all kinds of warnings on the DIC (“Service emissions”, “174 miles before 65 mph max”, “Limiting power to 40 mph”, “Power steering assist deactivated”, the engine stopped, “Put in neutral to restart engine” (interesting since I was now coasting at 30 mph with flashers on, trying to make an unpowered right lane change in front of several 18-wheelers in order to get onto the shoulder, and would have to apply the brake in order to restart). I put it in neutral, hit the start button, all the instrument lights and information displays turned off. Putting it in neutral did help increase the coasting distance. After I coasted safely off the road, I checked the codes.

P0336 CS pos sensor perf

P0087 Fuel Rail Low Pressure

P0700 Trans Control Module

P228A-B Fuel pressure regulators 1-2 force engine shutdown

U0073 CAN bus Communication

U0074 Ctrl mod comm bus B off

U0076 Ctrl mod comm bus D off

U0129 lost comm with brake system control module

U029D-E lost comm with NOx sensor 1-2 module

U02A1 (no definition in reader)

U0100 lost comm with ECM

Was able to continue home. Didn’t use cruise control, used 5th and 6th gears. Took it to dealer. They had GM field tech look into it. Based on others vehicle problems and solutions, they are going to replace the engine wiring harness. Should be ready in a week. We’ll see what happens.
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Sounds like the same thing mine is doing. Let us know what happens.
 
I had the same problem as everyone else. Reduce speed notice if def wasn't filled, but my def was full. My truck was at approximate 18,000 miles and I was was on a 700 mile trip towing a trailer when it started. The only thing I could say is that the truck never went into reduce speed mode even though I drove further than what the display was indicating. Upon returning from trip brought it into the dealer and they tested it, Def system had no problem and def fluid tested it was good. They had the truck for 4 days and then said parts needed to be ordered but told me I could take the truck until parts came in, which was three days later. At this point truck went into the shop, new parts put on but did not fix the trouble. The mechanic now said that he felt the problem had something to do with the catalytic converters but he could not order them he had to do what the engineer wanted done first. To make a long story short bottom the dealership had my truck for over 38 days. They ended up replacing two catalytic converters and other parts. They told me between parts and labor this job would have cost around 15 thousand dollars but it was a warranty job. I have had the truck now for a couple of months and have not had any problems, the truck is running great, but I am still going to probably look into the lemon law because of the problems and the length of time they had the vehicle. It sucks paying on a truck when you don't have it even though they did give me a loaner after two weeks in, Nissan Rogue......
 
Sorry about the delayed follow-up.

After I took the truck back to the dealer (see my previous posts on Oct 3, 2022), Chevy finally figured out what the problem was and has fixed it (so far so good). The big key to diagnosing the problem was that, during that week-long trip pulling a travel trailer, every time one of these incidents happened, I would stop, plug in a code reader and document all the codes that were set at that time. I did NOT clear any codes because I wanted Chevy to be able to read them all). I also noted the date and time, ODO reading, and description of what was happening before, during, and after the problem. In every case, once I was done documenting everything, I would fire it back up and would drive off as if everything was "normal".

In the past when I took the truck in, the newer, less experienced techs (the older, more experienced techs had opted for early retirement during and after COVID) would read a "faulty xyz" code and then replace that module or sensor because the computer said so. Next time in, with a smattering of different codes, they would interpret the codes and replace what the computer told them to replace. They just kept replacing parts, but the problems would come back several hundred miles later.

This time when I took it back to the dealer, I had a three page chronological log of all the codes and symptoms that had happened over that trip. Their response this time was to escalate the problem to the regional tech (probably more experienced than the techs at the dealership). The regional tech determined that the problem was the engine wiring harness. Modern cars are computer driven and have all kinds of modules and sensors networked together. Unlike your office network, your car has no real central computer. Just a bunch of independent modules getting the data they need from other modules and sensors and then sending data to other modules and actuators over the common network. If there is a problem with the communications network (wiring harness) then it's real easy for a module to read erroneous data. It then responds to the bad data incorrectly and the snowball starts growing (the classic "garbage in, garbage out").

Since the wiring harness replacement, everything has been working well.
 
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When I hear these horror stories about a whole bunch of CEL codes being illuminated on people’s trucks I immediately think either wiring harness issues (ie: damage) or bad grounds! Glad they finally got yours sorted out from the sounds of it. Did they say if the harness was damaged at the factory during installation or was it something else that caused damage (a rub through)?
 
They didn't say anything about it. I do remember hearing about or reading about a tech bulletin regarding something about a wiring harness issue, but I don't remember where it was. The truck has had the trans rebuilt, then replaced with a rebuilt trans, then replaced with a new trans (mechanical problems inside the trans), and had the engine pulled to replace what the computer called a "bad" crankshaft position sensor. Lots of opportunities to damage wires.

My theory is that it's a manufacturing QC issue. Anything manufactured during COVID was potentially done by "newbies" because the regular workers were getting sick and being quarantined. The workforce gets depleted, so they hire a lot of new, inexperienced workers and QC goes out the window. My son is in the HVAC industry. They usually have very few problems with new equipment. However, they had a huge increase in problems with equipment that was manufactured during COVID that had to be pulled out and sent back. Since then, the QC on the HVAC equipment has improved and is back to where it was before COVID.
 
That would do it, if they pulled the engine… Lots of potential for damage. There were some with pinched main wire harnesses right from the factory, that’s why I was curious. There is a link on GMTechlink somewhere about it. At the end of the day, if it’s finally all resolved that is great news. It’s just too bad you had to put up with so much to get there….
 
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