To perform a tug test, first make sure that the engine is on, the transmission is in a low gear, and the parking brakes are set. Then try to pull forward. The parking brakes should prevent the vehicle from doing more than just slightly rocking back and forth. If you see the wheels actually roll, it means the vehicle has failed the tug test. This counts as a major defect. The vehicle cannot be legally driven on the road until this defect is repaired.