An exhaust catalyst is charged with oxygen until saturated. A given first rich air/fuel ratio (LAM1_SP) is set in a combustion chamber of a cylinder. A first oxygen storage capacity value (OSC1) is determined depending on the measured signals (MS1, MS2) from first and second exhaust probes. The exhaust catalyst is charged with oxygen until saturated. A given second rich air/fuel ratio is set in the combustion chamber of the cylinder. A second oxygen storage capacity value (OSC2) is determined depending on the measured signal from the first and second exhaust probe. A corrected oxygen storage capacity value (OSC_COR) is determined depending on the first and second oxygen storage capacity values (OSC1, OSC2).