or set back by 1 the counter 14 is actuated upon each change of the lambda probe 5 arranged in front of the catalytic converter 2 and is thus clock controlled by it at a first multiplication point 15 the count is multiplied by a proportionality constant provided by unit 18 and having a value of 0 5 several 100 ms probe change of the first lambda probe whereby an absolute hold time th roh is determined the hold time th roh thus obtained is evaluated at a second multiplication point 16 with a weighing factor wf which is located in a stored characteristic field 17 as a function of the load and of the speed of rotation n of the engine 1 the hold time th thus obtained is fed as control variable to the controller 8 of the system 11 for the adjustment of the control system 11 as an alternative to the determination via the characteristic field 17 the weighting factor wf can also be determined as a function of the period of the first oxygen measurement probe 5 for this purpose the measured period p ls5 of the first oxygen measurement probe 5 is divided by a constant p lls5 which corresponds to the period of the first probe 5 upon idling by comparison with the characteristic fields in which the weighing factor can assume maximum values of 1 the actual disturbance is now smoothed independently of its size because a kind of self amplification is achieved by the larger factor the hold time thus obtained is also fed to the controller 8 for adjustment of the control system 11 with reference also to fig4.