To operate an internal combustion engine, a specified forced stimulation is applied to an air ratio as the basis for a target value of a lambda controller. In diagnostic operation, a diagnostic function is used to identify a probe error of the exhaust gas probe, and a value of the measurement signal is recorded as a start value in chronological correlation with an edge of the target value curve of the lambda controller and the current value of the measurement signal is recorded as an end value after a specified first time duration. The start and end values are used to determine whether a filter error or a dead time error of the exhaust gas probe exists. The first time duration is specified such that start value/end value difference for a filter error differs start value/end value difference for a dead time error by at least a specified difference value.