The function of a respiration system, with a patient port (1) with connected inspiration branch (3) and expiration branch, is checked. A rebreathing line (9) connects the inspiration branch to the expiration branch. A reservoir (25) is connected to a reservoir port (27) in the rebreathing line. An actuatable control valve (29) is provided in the rebreathing line between the expiration branch and the reservoir port. A pressure sensor (33) is connected to the rebreathing line. A control unit (39) is connected to the control valve and to pressure sensor. The process includes closing the control valve for a preset inspiration time and opening it for a preset expiration time. The value sent by the pressure sensor is detected with the control valve opened during the expiration time and compared with a preset first threshold valve. An error message is generated when the value sent exceeds the first threshold value.