A method and a device for diagnosing an ascertainment of a performance quantity of an internal combustion engine. Within the scope of a first diagnostic process a first value which is characteristic of a first performance quantity of the internal combustion engine ascertained at a first point in time after switching off the internal combustion engine is compared to a predefined value, and an error is identified when in the comparison it is detected that the absolute value of a first difference between the first value and the predefined value exceeds a predefined first threshold value for the first point in time. The predefined first threshold value is predefined to be smaller the longer the time since the internal combustion engine has been switched off. An identified error is cleared in a subsequent second diagnostic process when, for a second difference between the first value and the predefined value for a second point in time, it is detected within the scope of the subsequent second diagnostic process that the absolute value of the second difference does not exceed a second threshold value predefined for the second point in time. An error identified within the scope of the first diagnostic process is cleared within the scope of the subsequent second diagnostic process only when the predefined second threshold value does not exceed a predefined limiting value. The predefined limiting value is formed as a function of the predefined first threshold value.