An internal combustion engine has an exhaust gas sensor upstream of or in an exhaust gas catalytic converter, and having a measurement signal corresponding to a residual oxygen content of the exhaust gas flowing past the exhaust gas sensor. For operation, an air ratio is specified, a specified forced stimulation is applied thereto, and at least one actuator that influences the mixture is set by appropriately controlling the actuator. In correlation with a rising edge of the forced stimulation, a first maximum value of the air ratio is determined according to the signal profile of the exhaust gas sensor measurement signal during a first measurement duration, and a second maximum value of the air ratio is determined according to the exhaust gas sensor measurement signal during a subsequent second measurement duration. A first sensor characteristic value is determined based on the first and second maximum values.