Long time listener, first-time caller. I’m responding to your plea for new Piston Slap questions. I purchased a gently-used 2008 GMC Yukon Denali AWD a couple of months ago. Other than its appetite for fuel, the only negative is that it has 141,000 miles. I believe the previous owner changed the transmission fluid at 100,000 miles (Carfax shows that the transfer case fluid was changed at this point, and I can’t imagine doing that and not doing the transmission). The fluid was relatively clean but I changed out several quarts via the dipstick tube using a fluid extractor after I purchased the vehicle, replacing them with the specified Dexron-VI. I believe the fluid level is correct but it’s difficult to read.
On a recent road trip, the 6-speed automatic (6L80E) transmission stumbled during the 2-3 shift while driving through the mountains and went into a failsafe mode. The check engine light came on. I pulled over, turned the ignition off and on again, and the truck operated normally. The CEL remained on for the next several ignition cycles. When I called OnStar to obtain the fault code, they could not retrieve it because the CEL was no longer on.