of signals from the exhaust gas probes belonging to the exhaust gas purification unit on the basis of these signals the oxygen storage capacity of the individual catalytic converter is determined despite the lack of exhaust gas probe between the individual catalytic converter and the main catalytic converter application_number US 66600306 A description fig1 shows an exhaust gas system in a y configuration associated with an internal combustion engine 1 the internal combustion engine 1 has two cylinder banks 2 3 an exhaust manifold 5 is assigned to the cylinder bank 2 and an exhaust manifold 4 is assigned to the cylinder bank 3 for cleaning of the exhaust gas generated by the cylinder bank 2 3 respectively furthermore the exhaust manifold 4 includes an individual catalytic converter 6 and the exhaust manifold 5 includes an individual catalytic converter 7 for cleaning of the exhaust gases generated in the respective cylinder banks 2 3 downstream exhaust pipes 8 9 of the exhaust manifolds 4 5 converge into a common exhaust pipe 10 the common exhaust pipe opens into a main catalytic converter 11 the main catalytic converter 11 serves for removing pollutants from the exhaust gas which can be only inadequately removed with the individual catalytic converters 6 7 for example the main catalytic converter 11 can be implemented as a nox storage catalytic converter 11 and the individual catalytic converters 6 7 can be implemented as three way catalytic converters furthermore the exhaust gas cleaning system has an exhaust gas probe 12 upstream of a first individual catalytic converter 6 an exhaust gas probe 13 upstream of a second individual catalytic converter 7 an exhaust gas probe 14 between the first individual catalytic converter 6 and the main catalytic converter 11 and an exhaust gas probe 15 downstream of.