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Are all your fuses seated tightly? Ubec bolts for the fuse block tight? https://gm-techlink.com/?p=13437 When I had my 2020 TB I had the same thing happen. All of the fuses in the panel seemed to be loose. I was able to press each on in a few mm. After that truck started right up. I made an appt at the dealer and they didn't find any codes. The Ubec bolts were fine (green indicator was up) but they still checked everything in the TSB and retightened etc. I had the truck for at least 6 months after the dealer looked at it and didn't have the issue again. While your codes aren't listed it could be the cause.

Did you reset the computer (trip and 25/50/400 mile)? Trucks get a lot of idle time before they are purchased.

Out of all three options from GM I would only go with the 10 speed.

Sounds like your dealer lost or gave your supplemental manual to someone else and wants you to pay for replacing it..... You should get all of the paper manuals with your truck for FREE.

When I had my 6.2 I tried to run 93 if not then 91. I did notice slightly better performance and higher MPG.

Here is a pic of my MX4

I have a BAK MX4 folding and mine are exposed. https://bakindustries.com/bakflip-mx4 They show one of their rolling covers on an AT4. https://bakindustries.com/rolling-covers

The oil visually looks good? Doesn't look like it is mixed with coolant? Def. an odd problem. I haven't heard or seen that before.

I had the USB ports and SD card reader stop working in the center console in my 21 AT4. This was after 3 days of owning. The connector had come undone behind the dash. Probably wasn't secure from the factory. You could check all your fuses and make sure that isn't the problem. When I had my 2020 TB it died in the parking lot at my business and almost all of the fuses were loose. after pressing them all tight into their sockets the truck was fine. dealer couldn't find any thing wrong. they also checked the ubec bolts but they were good too. ran fine for 6 months after that....

I have had my AT4 for almost 2 months. I use the MF tailgate multiple times a week. Just being able to drop the inner gate is helpful for loading and unloading stuff in the bed. The step is nice as well. I haven't used it in the load stop positions yet. I don't leave a hitch in as I don't like the look, don't want it to rust to the receiver and I value my shins. I also don't tow very often. If you are loading stuff into the bed with ramps make sure you read the manual. Either need to lower inner gate or distribute load across whole tailgate (manual explains better). The MF tailgate will not power close. I thought the MF tailgate was a gimmick until I used one.

That is good on the weight, I would have thought more. I will be getting black AT4 and GMC badges in a couple weeks and deleting the chrome Sierra on mine. The chevrolet done in black or another color really make it pop, especially on white.

I drive a truck as I like trucks, I am not a car guy. I like to be in a bigger and higher up vehicle as there are a lot of shitty drivers. With that said I do use my truck as well for truck stuff so if I can get better economy it is an extra benefit for me. The response of no one drives a truck for fuel economy is not helpful on a thread asking about going from the 6.2 to the 3.0 as fuel economy would be the main reason for doing so......

black badges and black chevrolet for the tailgate. To me the chrome blends in too much with the white. I am not a fan of chrome though How much weight did those bumpers add?

Trim and tires effect this. Non AT4 and especially non Duratracs would give about 2-3 more mpg. Those 2 items add a lot of wind drag and rolling weight for the tires. With that said I am a direct comparison as my TB had duratracs as well. I will add that my lifetime went up to 22.4 yesterday with basically just small town driving yesterday. I will add that I drove my 6.2 like it deserved to be drove at times. That motor was fun. As the 3.0 is just getting broken in I have started to get on the pedal heavier. Regardless my lifetime right now is slightly more than a 7 mpg gain. Diesel around here is the same (or cheaper) than premium. 160-170 extra miles per tank. Roughly saving me $20-25 per tank. I believe that the silverado gets slightly better MPG as it is a little more aerodynamic. I will also add that the 10sp transmission is a game changer. smooth. always seems to be in the right gear. definitely helps with the economy. I have had 6 and 8 sp GM transmissions, tundra 6 sp, ram 8 sp and by far the 10 speed is the best 1/2 ton transmission I have driven. I am curious to see what 14k of mileage looks like on the 3.0 for more accurate comparison.

These are the ones I got: 84742428 https://accessories.gmc.com/product/crew-cab-sport-step-assist-steps-in-black-with-sierra-script-84742428?year=2021&make=GMC&model=Sierra 1500&modelId=548&body=Crew Cab Pickup&bodyId=22&wheel=Short Box&wheelId=651&trim=AT4&trimId=5102&drive=4WD&driveId=8&engine=3.0L L6 DIESEL&engineId=17881&bodyNumDoors=5&categoryId=12062 GMC accessory page calls them sport steps however when you go through the build site they are listed as Off-Road. The high clearance steps are too high and narrow for my son to use. I had the off road steps on my TB so I knew my son would be able to use them.