A dishwasher includes a controller controlling a cleaning cycle, a screen system for filtering a wash fluid, a circulation chamber and a collection chamber in fluid communication with the circulation chamber via a screen arrangement, a circulation pump for circulating the wash fluid and a drain pump for pumping out the wash fluid. During the cleaning cycle, to detect blockage in the screen arrangement, a degree of transmission in the circulation chamber filled with wash fluid is determined with an optical turbidity sensor in a first measuring step. The wash fluid is then pumped out of the collection chamber and the degree of transmission in the circulation chamber is again determined in a second measuring step. The change in the degree of transmission from the first measuring step to the second measuring step is then evaluated.