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Let me preface by saying I am trying to describe the issues I had as best as I can so so sorry if I get long winded.

2017, 2.8L obviously, 75k on the clock. 4000+ engine hours. Purchased 1/31/2018. East Texas, north Louisiana area. It gets hot.

I just recently got fired from my job so at the moment (the past few days) I've been delivering food. Lots of stop and go traffic, lots of idling as I pick up orders, but that's the way I've always driven this truck. My previous job was in outside sales so I was making sales visits 5 days a week, 12 months a year.

Anyway let's fast forward to today. Worked from 3:30-7:30 with no issues with the truck/transmission. Go home from 7:30-8, then go back and start working again at 8. That's when all the problems started.

Instance 1: delivering to an apartment complex. Thought I was at the right building so I pulled into a spot and parked, left the truck running. Was the wrong building so I get back in the truck, reverse, then put in drive. Idling through the parking lot trying to ensure I find the correct building. Try to give it a little gas and the transmission acts like it's in neutral, won't respond to the throttle at all except the RPMs rev all the way up to 4k. I thought maybe I had somehow bumped the shifter into neutral, but it was in D according to both the position of the shifter and the dash indicator. All of a sudden the trans drops into gear and bucks forward and drives like normal. Make delivery, carry on about my way, no issues.

2nd instance: At a stoplight. Light turns green, I try to progress into intersection. Same thing happens again. No throttle response, RPM revs up, truck suddenly drops into gear and responds to throttle. This time the buck was so severe it felt like I had been rear ended (and I know what it feels like to be rear ended in this truck because I have been rear ended in it). This was the worst of the bucking.

3rd instance: drop final delivery of the night about 10pm. Shift out of park and into D and make a right turn. Same stuff is happening again but this time it's not quickly dropping into gear. Pull to the side, put the car in park. Shift back into drive, issue still has not corrected itself. Pull back to the side. Put in park, turn the ignition off. Wait a few seconds, turn the truck back on. Now the truck is driving normally. Proceed to the next intersection. Attempt to make a left turn across 2 lanes, same issue occurs. Pull into the turn lane in the middle of the road, and come to a complete stop. Didn't shift into park this time, just left it in drive. RPMs drop to 1500 so I attempt to accelerate again and now the truck is driving normally.

Had about a 15 minute drive home with no further issues. It did feel more sluggish when going from a complete stop to accelerating than it usually does. Only at this point did I think to check the transmission temperature, which was 138F. I'm not sure what the ambient temp was at that time but as I write this it's 47F, and it wasn't much, if any, warmer at 10pm. That seems pretty high for driving an empty truck, nothing hitched up, on a cool night.

Some other notes:

-The ATF has never been flushed or replaced because I have (stupidly) believed the owner's manual that says it doesn't need to be.

-I have never experienced any other transmission issues at all. No shuttering, none of this bucking, no hard shifts, nothing. In fact, when people would ask me about the truck, I would actually rave about the transmission because it fit my driving style and habits. Maintained speed to my satisfaction, never noticed it searching for gears, etc.

-About 30k miles ago I had a NO2 (or maybe it was NOx? The service paperwork is in the truck) sensor go out that was replaced under warranty. At the time of that service the dealership reccomended a new ECM flash that I did accept and paid for. I definitely noticed the truck re-learning my driving habits as well as a drop in fuel efficiency over that time period but that bounced back gradually over time.

I'm just at a loss, because if I need a new transmission it's going to devastate me. This obviously seems like something that needs to be addressed immediately but I'm not sure how I will be able to pay for it, being in-between jobs. Just looking for information about whether or not this is a common issue so I can have that in my back pocket when discussing options with a dealer. Is the truck viable to even drive around to different dealerships until I find one that will work with me or do I risk destroying the transmission further by driving it? Trying not to panic.
 

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Update: I drove it again this morning and was able to replicate the issue. Drove to a gas station that is about 10 minutes away each way. The first time it happened today it was just a slight pause/lurch after going from reverse into drive. The second instance today was at a stoplight making a right turn on red, so full stop to throttle, that one was more of a hard buck like I was experiencing last night but the engine wasn't bogging down.

In this 20 minutes of driving the transmission temp got up to 127F on a day where the ambient temp is 56F. It's easy Texas so it is not like I'm driving up severe grades or anything. Could this be an issue with a TOC? Is there a chance that the TOC is leaking water into the transmission because that's kind of what it feels like and I know GM has had an issue with TOC lines breaking and doing exactly that (not on this transmission, but others)...
 

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Update 2: 1st gear seems to have just given out today. Was at a stoplight attempting to make a right turn, and 1st gear just wouldn't engage, in fact the truck rolled backwards. Transmission temp was 143F last I checked before turning the truck off. When it rains it pours I guess...
 

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That’s not good. I know there is a somewhat known issue with the torque converter going out on these so that could be the issue by he sounds of it possibly.
 

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@emurdo4 Is your truck 4WD? If so, it could be the same issue I had with wire chaffing on your transfer case module. The difference being your truck is jumping the transfer case into neutral and not 4-LO like mine did. To isolate:

1. Disconnect the negative battery cable for about 2 mins (this resets all systems). (Your transfer case should be in 2WD prior to starting)
2. Reconnect and crank your truck. Do not put in gear or anything, just crank and let idle.
3. Immediately after cranking, pull your transfer case fuse, its located under the hood. I can't remember the number, but it is listed in the diagram sticker on the cover or in the owner's manual.
4. Once you pull the fuse, your module will not light up (4WD control knob). Once pulled, try to drive your truck as normal and see if the issue persists.

If you do not re-encounter the issue, it is with your transfer case module/ wiring. If the issue persists, I would bet it is the transmission control module, which is relatively cheap all things considered.

I hope this helps, let us know if it works.
 

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I have a 2016 LT with 116,000 miles. After a 2,100 mile trip in summer 2019, the truck started shifting harshly and I noticed a smell after driving.
I took the truck to the dealer and found out that the trans fluid level was over half gone from a leak in the rear seal. The leak flowed into the muffler and burnt off....NO PUDDLE....ever. In addition, there’s no alert light on the dash for low trans level and no dipstick to check.
In any event, I had to buy a brand new $4300 transmission because the seal isn’t covered by warranties. I’ve also had to replace the torque converter at 80,000 miles.
 

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I have a 2016 LT with 116,000 miles. After a 2,100 mile trip in summer 2019, the truck started shifting harshly and I noticed a smell after driving.
I took the truck to the dealer and found out that the trans fluid level was over half gone from a leak in the rear seal. The leak flowed into the muffler and burnt off....NO PUDDLE....ever. In addition, there’s no alert light on the dash for low trans level and no dipstick to check.
In any event, I had to buy a brand new $4300 transmission because the seal isn’t covered by warranties. I’ve also had to replace the torque converter at 80,000 miles.
That is absurd, I am very sorry to hear! Was there any early indicators that you noticed or may have dismissed? The tranny is what I worry about the most on our trucks. I generate a ton of heat towing the boat all summer long, I am worried that this summer will be the one to have an issue.
 

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That is absurd, I am very sorry to hear! Was there any early indicators that you noticed or may have dismissed? The tranny is what I worry about the most on our trucks. I generate a ton of heat towing the boat all summer long, I am worried that this summer will be the one to have an issue.
The only thing I may have missed was the weird smell, thinking it was someone else’s vehicle or that I had hit some plastic that stuck to the muffler.
 

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@Antman92 Just to clarify, the rear main seal was leaking onto the DOC/ down pipe which was burning off the fluid?
What a bummer! BS on this 'sealed transmission' with no dipstick!. Given your seal issue, how is one to know that the level is dropping without going up on a lift or crawling underneath to unscrew the plug to see if fluid weeps out?
Who thinks of these ideas? OK, my dipstick was $130.00 and the install during a transmission flush was an hour of labor, I have a reasonable method of checking the fluid. But would we pay an additional say $50.00 on the MSRP? I sure would.
Truly sorry this has happened to you, although it sounds like you got some good mileage on your truck, except for the torque convertor. I don't know enough about the TC to know if it involves the rear seal.
 

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What a bummer! BS on this 'sealed transmission' with no dipstick!. Given your seal issue, how is one to know that the level is dropping without going up on a lift or crawling underneath to unscrew the plug to see if fluid weeps out?
Who thinks of these ideas? OK, my dipstick was $130.00 and the install during a transmission flush was an hour of labor, I have a reasonable method of checking the fluid. But would we pay an additional say $50.00 on the MSRP? I sure would.
Truly sorry this has happened to you, although it sounds like you got some good mileage on your truck, except for the torque convertor. I don't know enough about the TC to know if it involves the rear seal.
Would you please link to your dipstick? I just changed mine at ~40K miles, but I'll be doing it again in a couple of years and would like to retrofit. No, I didn't review any of your other posts, but would you kindly indulge me?
 

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Would you please link to your dipstick? I just changed mine at ~40K miles, but I'll be doing it again in a couple of years and would like to retrofit. No, I didn't review any of your other posts, but would you kindly indulge me?
Well, if I understand your question, you want to know which dipstick I had installed. It was the one recommended on coloradofans.org under the transmission dipstick thread. It is a Lokar XTD36L80FM36 which I bought trom www.summitracing.com/ for $132.95. It fits well after a few adjustments as described in the thread. Be for sure that I didn't do the install, but an ASE tech at the shop I use did it during the flush and refill of my transmission at 7,700 mi to clear out any manufacturing debris.

Good luck! And I hope that the above answers your query.
 

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Yes, Streamtrip, it did. And many thanks for saving me the detective work on finding the dipstick. Your "adjustments as described in the thread" aren't in this thread (unless I'm overlooking them), but you have already done much of my homework for me, and I'll find that thread. Thanks again, Streamtrip.
 

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Yes, Streamtrip, it did. And many thanks for saving me the detective work on finding the dipstick. Your "adjustments as described in the thread" aren't in this thread (unless I'm overlooking them), but you have already done much of my homework for me, and I'll find that thread. Thanks again, Streamtrip.
The specific thread is Trans. Dipstick GOOD News for 2nd Gen. Owners-MUST READ!, The thread itself is interesting because it starts with a Colorado gasser transmission, but later on it migrates over to the Duramax. Page 5 is where you really want to be.
 

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What a bummer! BS on this 'sealed transmission' with no dipstick!. Given your seal issue, how is one to know that the level is dropping without going up on a lift or crawling underneath to unscrew the plug to see if fluid weeps out?
Who thinks of these ideas? OK, my dipstick was $130.00 and the install during a transmission flush was an hour of labor, I have a reasonable method of checking the fluid. But would we pay an additional say $50.00 on the MSRP? I sure would.
Truly sorry this has happened to you, although it sounds like you got some good mileage on your truck, except for the torque convertor. I don't know enough about the TC to know if it involves the rear seal.
Interesting...I’d definitely pay “extra” for a dipstick and the low-level warning light that aren’t current present.
 
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