First Sierra purchase, looking for a 2021 3.0 with the SLT package

[cherryred21duramax]

First Sierra purchase, have been a Ford truck owner and want to try something new.  I'm looking at a 2021 3.0 Duramax with the SLT package, preferably with 8" touch screen and Nav.  What should I look for when test driving used trucks?  I've driven 3 so far and they felt dramatically different, some had a lot of engine noise while others were super quiet.  One had a noticeable downshift while coming to a stop, while the others didn't flare when shifting.  Some had a very sensitive throttle (checked that they were all in the same mode, put in Normal before test driving) and others had the normal turbo lag you'd expect from a diesel.

 

I'm finding it odd that all of these trucks are between 99-120k miles.  Is there something specific that results in people trading these in around this timeframe?  Most were 1-owner, used personally, and CARFAX showed "engine/powertrain checked" or "emissions system checked" but doesn't show the actual repair.

 

Looking at both GMC dealerships and independent car lots, is there any benefit to going to a tier-1 dealership (such as Carmax) for warranty options?  I have never purchased a used car and bought the extended warranty.

 

Last, what's up with all the trucks having the Treble cranked to +12 on the radio (both Bose and non-Bose options)?  Every single one I drove was set that way by the previous owner.  Are the speakers lacking, or did I test drive 3 back-to-back with the dash tweeters blown?

[silveradosid]

90-120k is when you are going to start having transmission problems, even if it has been serviced the valve body wears out and next thing you know you have burnt clutchs and the cost of a rebuild, let alone the torque converter prolems

[newdude]

On 5/31/2026 at 11:47 AM, cherryred21duramax said:

First Sierra purchase, have been a Ford truck owner and want to try something new.  I'm looking at a 2021 3.0 Duramax with the SLT package, preferably with 8" touch screen and Nav.  What should I look for when test driving used trucks?  I've driven 3 so far and they felt dramatically different, some had a lot of engine noise while others were super quiet.  One had a noticeable downshift while coming to a stop, while the others didn't flare when shifting.  Some had a very sensitive throttle (checked that they were all in the same mode, put in Normal before test driving) and others had the normal turbo lag you'd expect from a diesel.

 

I'm finding it odd that all of these trucks are between 99-120k miles.  Is there something specific that results in people trading these in around this timeframe?  Most were 1-owner, used personally, and CARFAX showed "engine/powertrain checked" or "emissions system checked" but doesn't show the actual repair.

 

Looking at both GMC dealerships and independent car lots, is there any benefit to going to a tier-1 dealership (such as Carmax) for warranty options?  I have never purchased a used car and bought the extended warranty.

 

Last, what's up with all the trucks having the Treble cranked to +12 on the radio (both Bose and non-Bose options)?  Every single one I drove was set that way by the previous owner.  Are the speakers lacking, or did I test drive 3 back-to-back with the dash tweeters blown?

 

 

Yes.  They don't want to pay for the oil pump belt job due at 150,000mi.  Or they had 10 speed issues like valve body.  Oh and emissions sensor problems.  

 

Bass to 12?  Take it back to neutral and you'll see why lol.  Especially on the non Bose trucks.  

Edited June 4 by newdude

[cherryred21duramax]

On 6/3/2026 at 6:22 AM, silveradosid said:

90-120k is when you are going to start having transmission problems, even if it has been serviced the valve body wears out and next thing you know you have burnt clutchs and the cost of a rebuild, let alone the torque converter prolems

Ouch, that's why I left Ford.  Both my 10R80 and a friend's 10R80 had valve body problems.  And on his, he barely towed.  Mine eventually got the CDF drum, but it never drove right, so I ended up selling at a loss.

 

10 hours ago, newdude said:

 

 

Yes.  They don't want to pay for the oil pump belt job due at 150,000mi.  Or they had 10 speed issues like valve body.  Oh and emissions sensor problems.  

 

Bass to 12?  Take it back to neutral and you'll see why lol.  Especially on the non Bose trucks.  

So it sounds like a thorough check of the transmission (assuming these have a "fast relearn" mode) will be a good idea.  Or look for one that's had its 10-speed recently rebuilt.

 

Haven't touched the radio settings, but yes - almost every truck had bass at or near +12 and treble at +12.  Felt odd as on Fords and Rams, especially with premium audio, the door panels would be shaking with such settings.

 

Goal is to tow a travel trailer cross country and use the truck for long trips.  No in-town driving or local use, I have a car.  I see some with ultra high mileage (160-200k) going for the same price as lower mileage units, would you think those may be a better buy?

 

Here is an example with higher mileage (161k): https://www.alwaysmotors.com/pre-owned-cars/detail/2020-GMC-Sierra-1500/1521482

And here's one with lower mileage (113k) for the same price: https://www.shawnsmotorcredit.com/vehicle/used-2021-gmc-sierra-1500-slt-4x2-4dr-crew-cab-58-ft-sb-5195481

Here's one with under 100k but priced insanely high: https://www.group1auto.com/inventory/used-2021-gmc-sierra-1500-slt-rwd-crew-cab-3gtp8det9mg411755/

And one I'm almost hesitant to even look at, it's way cheap: https://www.taylorgrubaughchevy.com/used/GMC/2020-GMC-Sierra-1500-e7201fe2ac1810998925e011ba32db79.htm

 

Given trucks like the above, would the 161k version with recent repairs be a better buy than the lower mileage ones?  I could care less about what the odometer says, I would rather have one that is more reliable and doesn't leave the trailer stranded at a campsite.

[silveradosid]

all depends on if you can get the service records

[cherryred21duramax]

1 hour ago, silveradosid said:

all depends on if you can get the service records

 

Would the best option be to use a local dealer?  I'll be flying into town, most dealers I've called want to charge to do a used car inspection.  Can get expensive if I'm comparing 3+ trucks on the same day, not to mention scheduling.

 

All I see on CARFAX reports is "Engine/powertrain checked" or "Emissions system serviced" and on a few, "Transmission replaced" or "Valve body replaced" which is concerning.  A lot of "Transmission Replaced" prior to 140k so I guess I'll have to plan for some extra cash in case of an emergency.

 

Is it normal for the 10L80 to downshift hard and flare when coming to a stop?  I've now experienced it enough (along with some rear end noise when test driving) that I don't know what's a normal 10L80 in non-sport or Tow-Haul mode.  Some trucks downshifted cleanly, others lurched and jerked as the RPMs jumped around even in Normal mode.

[silveradosid]

if it is jerking and flaring between shifts it need a valve body are even more

[cherryred21duramax]

1 hour ago, silveradosid said:

if it is jerking and flaring between shifts it need a valve body are even more

 

Thank you.  I ended up passing on that truck, it was $29k with 99k miles and local to me (Illinois).  Played that game once before, passed a much better condition vehicle for one that was closer.  $10k in engine work and $6k in transmission work later, it never drove right, and by that point the age & mileage made it worth close to nothing.  I now remind myself, a plane ticket and an Uber is a lot cheaper than buying the local truck with issues and having buyer's remorse.

 

Now looking at this for $26k: https://southernwholesale.net/vdp/23915469/Used-2021-GMC-Sierra-1500-SLT-Crew-Cab-Short-Box-2WD-for-sale-in-Trussville-AL-35173

And AutoCheck looks good: https://autocheck.dealercarsearch.com/Default.aspx?VIN=EPWlh9Ow3ceghogyxJFSf4bAycMno2xr4%2fK1GUasMDw%3d&DealerId=25052

 

This one from Carmax is high ($32k), but some people swear by the Maxcare warranty: https://www.carmax.com/car-detail/27952269

 

And this one for $33k seems insanely high priced, but it got a new engine at 120k, and no mention of transmission work except for fluid changes: https://www.alejandrocars.com/inventory/2021-gmc-sierra-1500-127941

 

Just trying to not repeat my first used car purchase, being so underwater I had to sell it to a high school w/vocational shop class because nobody would buy a recently repaired lemon.