A method for verifying at least one function of a medical functional device which is inserted in and connected with and pressed together with a medical treatment apparatus, and/or for verifying at least one function of this treatment apparatus, wherein at least one fluid communication is established between a hydraulic device or a pneumatic unit of the treatment apparatus and the functional device, wherein the method comprises the steps of: - building up a pressure by means of the hydraulic device or of a pneumatic unit within the fluid communication; - measuring a pressure prevailing within the fluid communication or a pressure change; - making a statement about the verified function of the functional device based on a comparison of the prevailing pressure or of the measured pressure change with previously saved values, threshold values, ranges or evolutions; - blocking treatment modalities and/or restricting treatment parameters of the treatment process taking into account a result of the verifying of at least one function of the medical functional device and/or of the treatment apparatus, wherein the result has been obtained by means of the present method; - conveying a predetermined feed volume (MAXFOERDERVOLUMEN) within the fluid communication with the substituate pump running in reverse direction against a non-return valve which is provided within a substituate channel, and - detecting a pressure change within the fluid communication, wherein the method comprises at least one of the two following steps: - detecting that the substituate pump conveys or conveyed forwards, if a detected pressure rise is higher than a maximum pressure rise (MAXDRUCKANSTIEG) or higher than a predetermined value of a pressure change (MAXIMALE DRUCKAENDERUNG) and/or - detecting a non-return function of the non-return valve, if a detected pressure drop is not equal to or higher than a maximum pressure drop (MAXDRUCKABFALL) or not higher than a predetermined value of a pressure change (MAXIMALE DRUCKAENDERUNG), wherein the medical functional device is a blood cassette, and wherein the treatment apparatus is a blood treatment apparatus, and wherein the method runs automatically. - building up a pressure by means of the hydraulic device or of a pneumatic unit within the fluid communication; - measuring a pressure prevailing within the fluid communication or a pressure change; - making a statement about the verified function of the functional device based on a comparison of the prevailing pressure or of the measured pressure change with previously saved values, threshold values, ranges or evolutions; - blocking treatment modalities and/or restricting treatment parameters of the treatment process taking into account a result of the verifying of at least one function of the medical functional device and/or of the treatment apparatus, wherein the result has been obtained by means of the present method; - conveying a predetermined feed volume (MAXFOERDERVOLUMEN) within the fluid communication with the substituate pump running in reverse direction against a non-return valve which is provided within a substituate channel, and - detecting a pressure change within the fluid communication, - detecting that the substituate pump conveys or conveyed forwards, if a detected pressure rise is higher than a maximum pressure rise (MAXDRUCKANSTIEG) or higher than a predetermined value of a pressure change (MAXIMALE DRUCKAENDERUNG) and/or - detecting a non-return function of the non-return valve, if a detected pressure drop is not equal to or higher than a maximum pressure drop (MAXDRUCKABFALL) or not higher than a predetermined value of a pressure change (MAXIMALE DRUCKAENDERUNG),