2013 F-150 3.5 Ecoboost. When towing in upper elevations, after driving for several hours and then stopping for gas engine throws a Turbo Underboost Code when reentering the highway. No loss of power is ever encountered. Resetting the code at lower elevation, it doesn't reoccur... until the next time I tow at elevation. Using premium gas makes no difference except for better gas mileage.