Procedure for the diagnosis of a two-wire field device designed to determine and/or monitor at least a process variable of a medium in a vessel, wherein, in a normal operating mode (M N), an input voltage (U N) is provided and an output current (I N) is output, and wherein, in a diagnostic operating mode (M D), a functionality of the two-wire field device is checked, wherein the diagnostic operating mode (M D) at least comprises the following procedural steps: Provision of a first diagnostic input voltage (U DE,1) and outputting of a first diagnostic output current (I DA,1) during a first time interval (t 1), Provision of a second diagnostic input voltage (U DE,2) and outputting of a second diagnostic output current (I DA,2) during a second time interval (t 2), Determination of the second time interval (t 2) starting from the first time interval (t 1) Determination of a first diagnostic output voltage (U DA,1) depending on the first diagnostic output current (I DA,1) and a second diagnostic output voltage (U DA,2) depending on the second diagnostic output current (I DA,2), and Verification of the functionality of the two-wire field device using the first (U DE,1) and the second diagnostic input voltage (U DE,2), the first (t 1) and the second time interval (t 2), the first (I DA,1) and the second diagnostic output current (I DA,2), the first (U DA,1) and the second diagnostic output voltage (U DA,2), using the input voltage (U N) and the output current (I N), characterized in that the second time interval (t 2) is determined from the first time interval (t 1) in such a way that a value of a first integral of a difference between the input voltage (U N) and the first diagnostic input voltage (U DE,1) over the first time interval (t 1) and a value of a second integral of a difference between the input voltage (U N) and the second diagnostic input voltage (U DE,2) over the second time interval (t 2) are essentially equal in terms of the absolute value. wherein, in a normal operating mode (M N), an input voltage (U N) is provided and an output current (I N) is output, and wherein, in a diagnostic operating mode (M D), a functionality of the two-wire field device is checked, wherein the diagnostic operating mode (M D) at least comprises the following procedural steps: Provision of a first diagnostic input voltage (U DE,1) and outputting of a first diagnostic output current (I DA,1) during a first time interval (t 1), Provision of a second diagnostic input voltage (U DE,2) and outputting of a second diagnostic output current (I DA,2) during a second time interval (t 2), Determination of the second time interval (t 2) starting from the first time interval (t 1) Determination of a first diagnostic output voltage (U DA,1) depending on the first diagnostic output current (I DA,1) and a second diagnostic output voltage (U DA,2) depending on the second diagnostic output current (I DA,2), and Verification of the functionality of the two-wire field device using the first (U DE,1) and the second diagnostic input voltage (U DE,2), the first (t 1) and the second time interval (t 2), the first (I DA,1) and the second diagnostic output current (I DA,2), the first (U DA,1) and the second diagnostic output voltage (U DA,2), using the input voltage (U N) and the output current (I N), Provision of a first diagnostic input voltage (U DE,1) and outputting of a first diagnostic output current (I DA,1) during a first time interval (t 1), Provision of a second diagnostic input voltage (U DE,2) and outputting of a second diagnostic output current (I DA,2) during a second time interval (t 2), Determination of the second time interval (t 2) starting from the first time interval (t 1) Determination of a first diagnostic output voltage (U DA,1) depending on the first diagnostic output current (I DA,1) and a second diagnostic output voltage (U DA,2) depending on the second diagnostic output current (I DA,2), and Verification of the functionality of the two-wire field device using the first (U DE,1) and the second diagnostic input voltage (U DE,2), the first (t 1) and the second time interval (t 2), the first (I DA,1) and the second diagnostic output current (I DA,2), the first (U DA,1) and the second diagnostic output voltage (U DA,2), using the input voltage (U N) and the output current (I N),