2026 GMC Sierra AT4 6.2L Owner Is Back to Dealer One Week After Purchase With a Highway Stutter, No Codes, and Five OnStar Calls Behind Him

A brand-new 2026 GMC Sierra AT4 powered by the 6.2-liter V8 engine is already back at the dealership just one week after purchase, suffering from a persistent highway-speed stutter with only a few hundred miles on the odometer. The truck’s owner, identified as SK, reported that the vehicle jerks and stutters during highway cruising without triggering any warning lights or storing diagnostic codes. The 6.2-liter engine pumps out 420 horsepower and 460 lb-ft of torque, mated to GM‘s 10-speed automatic transmission, and utilizes Dynamic Fuel Management (DFM) to shut off cylinders under light loads. Beyond the mechanical gremlin, the ownership experience got off to a rocky start when a dealership email typo required five separate OnStar calls just to get the mobile app working.

Fellow owners in a GM truck Facebook group weighed in with various theories, pointing fingers at cylinder deactivation, the transmission, or even a faulty fuel injector. One group member suggested an immediate AFM/DFM delete and transmission flush, though undertaking hardware modifications on a brand-new truck still protected by its factory warranty carries clear complications. Another owner noted that a bad fuel injector can easily mimic transmission symptoms from the driver’s seat. Furthermore, historical technical service bulletins indicate that light-load shudders occurring specifically during four-cylinder mode can sometimes be attributed to the interplay between DFM cylinder mode switches and torque converter lockup. With no active fault codes stored, the recommended course of action is for a technician to monitor live data during a highway test drive to determine whether the issue stems from the torque converter, cylinder deactivation transitions, or an engine misfire, thereby establishing a formal service paper trail.