The present arrangement provides a method for monitoring the performance reliability of a control unit and/or at least one sensor of a safety device for the protection of vehicle occupants, comprising the steps that a negative acceleration or velocity caused by an impact, in particular, be recorded; a value for the impact severity be derived from the recorded acceleration or velocity; and the value for the impact severity be compared to a predefined threshold value. If the value for the impact severity exceeds the predefined threshold value, an error-function signal will be output, which indicates that the proper functioning can no longer be ensured for the control unit and/or for at least one sensor. The present arrangement also relates to a diagnostic device for such a method.