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I have just turned 1400 miles and my check engine light is on, remote start (likely related) stopped working and On Star can only say they have found a 'fault code' that does not return to a specific issue. I've been struggling with harsh downshifts going down hill. I have read the threat re: hill decent but the transmission takes the revs to uncomfortable heights.

I am very pleased with the truck but not a trip to the dealer this soon. Any thoughts are appreciated. Any recommendations on Seattle-area dealers with experience on the baby Duramax would be welcome as my two local/Seattle GMC/Chevrolet service departments seem unfamiliar with the engine.

Thank you!
 

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Discussion Starter #3
I have just turned 1400 miles and my check engine light is on, remote start (likely related) stopped working and On Star can only say they have found a 'fault code' that does not return to a specific issue. I've been struggling with harsh downshifts going down hill. I have read the threat re: hill decent but the transmission takes the revs to uncomfortable heights.

I am very pleased with the truck but not a trip to the dealer this soon. Any thoughts are appreciated. Any recommendations on Seattle-area dealers with experience on the baby Duramax would be welcome as my two local/Seattle GMC/Chevrolet service departments seem unfamiliar with the engine.

Thank you!
 

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I have the same issues with really aggressive and unnecessary shifts
the truck will also downshift when I break when I highway speeds it's very annoying. I just barely the breaks and instant 3K+ rpms
 

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Had the same downshift/ rev issues w/ braking while in Drive on downhills. Tap the brakes lightly and it would downshift one gear (to around 2200), press the brake any harder and it downshifts another gear and revs are at a buzzy and uncomfortable 3K. Solution is trans in Manual mode on steep downhills, there will be no downshifts while braking. Also, you can select a lower gear on the +/- of the shifter to save brakes in Manual. There really needs to be a trans reflash for this issue... if there isn't one already. I'm only at 1500 miles on my '16 Canyon so still learning it.
 

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Do you guys have early build date trucks?
My truck was built 9/2016 and haven't had those experiences. 4,000 miles plenty of highway and towing only downshifts in tow haul mode
 

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Are you guys driving with the exhaust brake on? Because that is "tow/haul" mode and will cause downshift like that
 

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My 'downshifts' occur in Drive only w/ Tow/Haul off, makes no difference in Drive w/ Tow/Haul on, it still downshifts. That's why I expect there should be a trans reflash... that downshift is totally unacceptable and resulted in at least one 'no sale' according to my salesman... scared the crap outta the guy. Last night I learned that while in Drive w/ Cruise on on a long down a hill it will downshift at 2 mph over Cruise setting.... don't know what gear exactly it went to but RPM was 3500 before I canceled Cruise and switched to Manual. 3500 RPM is kinda buzzy and uncomfortable when it happens that fast.

I'll check my build date. So, IS there a reflash for this issue? Until then I'll just switch to Manual when encountering hills.
 

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3500 rpm is miserable in these trucks, I totally agree. I've only seen that rpm when towing heavy trailer at high speed and hitting a hill where the trans dropped a gear and sent the motor screaming and buzzing. Also makes no power up there, it shifts like it's a gas motor where the power is in the high Rpms. I'm getting a trans tune when I get my truck tuned, makes the truck hold gears better. Wish I knew the answer to your problem. Good luck.
 
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