In a breathing apparatus and a method for the operation thereof, a gas monitor monitors at least one gas and includes a pump that controls a sidestream flow from a patient circuit to the gas monitor via a sampling line having a first end connectable to the gas monitor and a second end connectable to the patient circuit. At least one pressure sensor measures a pressure at the sidestream gas monitor and provides a first pressure signal indicative of the measured pressure. A processor determines if a leakage is occurring in the sampling line based on the pressure signal.