For that model year Denali the 8 speed transmission were widely reported as having issues. I owned a '15 Yukon XL Denali with the 6.2L and 8 speed, I had the pleasure of putting 149K miles on it before it was totaled by another driver. Throughout that ownership I experienced the following major repair items:
48K miles 8 speed TSB flush and fill with new transmission fluid per GM
- This helped the hard 1-2 shifts and got rid of random drivetrain "speed bump" vibration allegedly from the torque converters
- Again common known issue amongst the 8 speed transmissions
55K miles driver alert/vibration sensor in the drivers seat that needed replaced.
65K miles A/C Condenser (known common issue across the K2XX platform)
78K miles failed AFM lifter #7 I believe; Despite having a Range AFM disabler installed since 29K miles. Repaired under GM CPO Powertrain warranty.
95K miles transmission would start to randomly slip while merging onto the highway. Of course could never duplicate for dealer or service adviser before GM powertrain 100K mile warranty ran out. I just continued to maintain the trans fluid and filter changes at the "severe" service intervals. Still had original transmission when it was totaled at 149K, but I was preparing for a rebuild/replacement in the future.
At the age and mileage of K2XX Denali's you're looking at likely most of the common issues have already been addressed. I would look for A/C condenser replacement in the service history, oil change intervals, and transmission service intervals. The A/C condenser can be a costly repair at the dealership because the front bumper cover has to be removed; Figure about $1,100-$1,300.
The potential AFM lifter issue is really a dice roll. Some vehicles experience it then there are a lot of vehicles that don't ever have the issue. It really doesn't add significant enough increase in MPG to out weight the potential long term reliability that disabling it does. A Range AFM disabler and frequent 4-5K mile oil change intervals are your best prevention unless you're willing to mechanically delete the AFM which is more involved but also worth it and beneficial.
The OEM MRC shocks/struts can be pricey to replace, but they're also pricey to replace on the GMT900 SUV's too. They definitely ride different on the K2XX gen SUV's compared to the GMT900 SUV's. That's subjective and perceived differently for different people. I happen to very much like the MRC in both the GMT900 and K2XX platforms. I'm only mentioning this because at the age and mileage truck you're looking at if the shocks/struts haven't been replaced it's likely coming up soon. And if they have been replaced I would check to insure it was with GM OEM MRC shocks/struts. Lot's of aftermarket knock-off MRC junk shock/struts out there.
The 8 speed transmission wasn't all that terrible, it drove "ok", it drove like many other previous GM SUV/Truck transmissions. Random hard clunky downshifts when coming to a stop, would sometimes delay surge when shifting out of park into drive on start up. When I had the 8 speed TSB fluid flush and refill with the new recommended GM trans fluid that did help quite a bit with some of those issues. IMO the 8 speed really shined in WOT shifts over putting around town.
However, the 10 speed doesn't have any of those previous common GM transmission issues. We now own a '19 Escalade ESV Premium Luxury with the 6.2L and 10 speed that is my wife's daily driver. I've driven it many times and on extended family trips and the 10 speed is light years better than the 8 speed and any other previous GM transmission for that matter IMO.
All that being said I'd still buy another K2XX Yukon Denali, but as mentioned by
@SpyShops212 try to get into an '18-'20 MY with the 10 speed transmission. Even if it has overall higher mileage in order to fit into your budget as long as it's been maintained and serviced and taken care of you'll be better off in the long run.
The only other major change that happened from '15-'17 is the audio/infotainment. I think it was either late '16 MY or starting with the '17 MY that GM added apple car play / android auto to the infotainment and changed the rear console input from RCA to HDMI.
They're great rigs and pricing has come down on them quite a bit in the last 6-8 months and interest rates are starring to creep down too.
Good luck in your search!
