2025 GMC Sierra 2500 6.6L Duramax Transmission Leak

[CRApex]

On 3/5/2025 at 7:08 PM, B.rad said:

I’m afraid that I may be experiencing a similar issue. My 2025 GMC 1500 Allison Transmission just hit 2,300 miles and the check engine light is on… the screen displayed - low transmission fluid. Not very happy about that as I’ve only made my first payment last week. I will be taking it to GM this weekend.

I hope I didn’t make a mistake going with GM. Do better!!!

The 1500 has a different transmission than the HD, its not the Allison branded version.

[Derek Smay]

My 2025 2500HD had same issue. They tried re-seal, and it leaked worse. GM authorized a new transmission, and all is well for now. 

[67mustang-man]

Just thought I'd add myself to this list. They replaced the seal and got it back worse than it was the first time around.

[chevyz98]

I joined the club today with a leak, A 2025 with 500 miles and the same leak. Dealer saying will be next week before they can get it in. Had a camping trip planned with the new truck but I guess we will have to use the 2016 LML.

Edited March 19, 2025 by chevyz98

[Chevyfan3]

On 3/19/2025 at 7:04 PM, chevyz98 said:

I joined the club today with a leak, A 2025 with 500 miles and the same leak. Dealer saying will be next week before they can get it in. Had a camping trip planned with the new truck but I guess we will have to use the 2016 LML.

Such a bummer this is still happening to 2025 trucks, GM really needs to get quality under control with these transmissions. 

 

Hopefully the reseal works out for you, some don't have luck with it and some do.

[67mustang-man]

On 3/19/2025 at 4:04 PM, chevyz98 said:

I joined the club today with a leak, A 2025 with 500 miles and the same leak. Dealer saying will be next week before they can get it in. Had a camping trip planned with the new truck but I guess we will have to use the 2016 LML.

No, they should make you a priority if you just bought it. They did for me.

 

By the way, the 2nd repair has so far been good. They said the 2025 transmission bulletin is incorrect and the seal needs to be hanging out so many mm so when they mate the transmission and transfer case they make a complete seal. Not installed flush

 

Fingers crossed so far no leaks.

[RDT-G23]

I just found drops on the bell housing inspection port and nearby on the front of the transmission casting.  I have ~2,200 miles on this 2025 DENALI HD 2500 (Duramax/10spd).  I have a friend who bought his GMC 2500 AT4 within a week of mine.  He has 2,700 miles.  Same issue.  I’ve had 7 diesel trucks since 2007, 5 power strokes, a 2012 DENALI HD and this 2025 DENALI Ultimate HD.  I have “0” tolerance for this kind of thing.  I paid over $100K cash for this truck in March.  I’ll drop it like third period French if it is any kind of issue.  I have the patience of a gnat.

Edited May 12, 2025 by RDT-G23

[GMCustomerService]

On 5/11/2025 at 8:14 PM, RDT-G23 said:

I just found drops on the bell housing inspection port and nearby on the front of the transmission casting.  I have ~2,200 miles on this 2025 DENALI HD 2500 (Duramax/10spd).  I have a friend who bought his GMC 2500 AT4 within a week of mine.  He has 2,700 miles.  Same issue.  I’ve had 7 diesel trucks since 2007, 5 power strokes, a 2012 DENALI HD and this 2025 DENALI Ultimate HD.  I have “0” tolerance for this kind of thing.  I paid over $100K cash for this truck in March.  I’ll drop it like third period French if it is any kind of issue.  I have the patience of a gnat.

We know this is the last thing you'd expect from a new 2025 vehicle. Our team is here to help, will you please email us at [email protected] with your VIN and dealership information? We hope to hear from you so we can learn more. By sending us a message, you consent to the information you provide being monitored and recorded by GM or those acting on GM’s behalf, subject to the GM Privacy Statement: https://www.gm.com/privacy-statement.

[Chevyfan3]

On 5/11/2025 at 9:14 PM, RDT-G23 said:

I just found drops on the bell housing inspection port and nearby on the front of the transmission casting.  I have ~2,200 miles on this 2025 DENALI HD 2500 (Duramax/10spd).  I have a friend who bought his GMC 2500 AT4 within a week of mine.  He has 2,700 miles.  Same issue.  I’ve had 7 diesel trucks since 2007, 5 power strokes, a 2012 DENALI HD and this 2025 DENALI Ultimate HD.  I have “0” tolerance for this kind of thing.  I paid over $100K cash for this truck in March.  I’ll drop it like third period French if it is any kind of issue.  I have the patience of a gnat.

 The drops on the inspection port are the front pump seal issue. Mine had this around 6500 miles. Trans has to come out for this issue to reseal the front pump cover housing.This has been going on since 2020 for this one, there is a TSB for it as well. When the tech put a camera up in the bell housing port we could see how much it has leaked as a good amount was sitting in the bell Housing.

[vscott]

I have a 2025 , 2500hd duramax with 7500 mi. Trans just started leaking. And I'm seeing oil puddle from under bell housing ( clear looking oil) and oil dripping from the tail of the transmission. (Red trans oil),Which I can see is running or blowing back from the front. Sure looks like the seal between trans and transfer case. Im hoping the dealership can get it replaced quickly,  since I have a trip planned in 2 weeks.