Twice the steering in sierra has locked up while turning. the first time i was taking a curve to the right, and the steering wheel wouldn't allow me to curve right. it would still allow me to turn left. after coming to a stop and parking the truck, the issues resolved itself. the second occurrence happened as i was taking a left turn. i had completed the left turn, and as the wheel was returning to the center(turning back to the right), the wheel locked. i was forced into the next lane and almost caused an accident. i had to use a lot of strength, and i eventually forced it. when it broke free, it made a loud pop sound. the dealership could not find the cause of the problem. i am afraid to drive the truck.