Dryer appliances and methods for operating dryer appliances are provided. A method includes flowing internal air from a drum chamber through an outlet assembly of the dryer appliance, and selectively actuating a valve to flow external air to the outlet assembly of the dryer appliance from external to a cabinet of the dryer appliance. The method further includes alternately sensing open flow pressure values of the external air being flowed to the outlet assembly from external to the cabinet when the valve is open and closed flow pressure values of the external air being flowed to the outlet assembly from external to the cabinet when the valve is closed.