A method is for adjusting a fuel/air ratio by means of an on-off controller as well as a diagnostic method in which a desired fuel/air mixture is regulated in accordance with a test signal of a lambda probe that is embodied as a jump probe. The switching point of the on-off controller is moved/adapted while the oscillation of the test signal of the lambda probe is analyzed regarding the amplitude and/or the asymmetry of the oscillation around the switching point at a constant control stroke. A desired value for the asymmetry or the amplitude of the oscillation of the test signal of the lambda probe around the respective switching point is predefined, the switching point of the on-off controller being moved such that the desired value is reached.