fig1 shows an internal combustion engine 1 to which charge air 3 is supplied via a charge air line 2 . from the internal combustion engine 1 an exhaust gas pipe 4 leads to an no oxidation catalyst 5 , which is followed in the flow direction 6 by an scr catalyst 7 . in the flow direction 6 after the scr catalyst 7 is arranged a particulate filter 8 . in the flow direction before the scr catalyst 7 , an arrow 9 indicates the metered addition of a reducing agent , for example an aqueous ammonia solution . slightly further upstream , an no x measurement device 10 is arranged in the exhaust pipe 4 and connected with a control device 12 via a signal line 11 . the control device 12 is also connected via line 13 with a shut - off element 15 arranged in an exhaust gas recirculation line 14 and via a further control line 16 with the internal combustion engine 1 for control purposes . in the exhaust pipe 4 before the no oxidation catalyst 5 is arranged an exhaust gas turbine 17 of an exhaust turbocharger 18 , by means of which in a known manner a compressor 19 in the charge air line 2 is driven to compress the charge air 3 . to test the function capacity , in particular the oxidation capacity , of the no oxidation catalyst 5 , for example in normal operation of the internal combustion engine 1 , the hydrocarbon concentration and / or carbon monoxide concentration in the exhaust gas of the internal combustion engine 1 can be raised . for this the control device 12 can specify a late post - injection to generate hydrocarbons , a homogenous charge compression ignition of the internal combustion engine or a partly homogenous internal combustion engine operation in order to provoke a desired increase in the emissions of hydrocarbons and / or carbon monoxide from the internal combustion engine 1 . by changing at least one operating parameter of the internal combustion engine 1 , the concentration of hydrocarbons and / or carbon monoxide upstream before the no oxidation catalyst 5 is raised at least to the value of the no x concentration upstream before the no oxidation catalyst 5 . in order to be able to raise the carbon monoxide emissions a desirable manner , by means of the control device 12 , for example the shut - off element 15 can be controlled such that the exhaust gas recirculation rate is raised to over 30 % in relation to the charge air quantity supplied to the internal combustion engine 1 . the recirculated exhaust gas quantity can also be raised in that a choke device , not shown here , is provided in the inlet - side charge air line 2 in order to increase the pressure difference between the exhaust gas and charge air 3 . alternatively or additionally , via the control line 16 the internal combustion engine 1 can be controlled such that the air / fuel ratio lambda can be lowered for example to below 1 . 05 and / or the injection pressure can be raised by for example at least 20 % or to at least 1200 bar . alternatively or additionally a shift in injection time for the internal combustion engine 1 can also be provided , for example to at least 20 ° crank angle but no more than 370 ° crank angle before the top dead centre ( ignition tdc ). also alternatively or additionally a lowering of the compression ratio , in particular by at least 20 %, and / or a change in valve opening times can be provided . such a change in operating parameters ensures that large quantities of reducing agent , in particular carbon monoxide , are generated in the exhaust gas flow of the internal combustion engine 1 and oxidised on the no oxidation catalyst 5 by means of the no x contained in the exhaust gas flow . the change in no x concentration due to oxidation of the carbon monoxide in the exhaust gas flow downstream of the no oxidation catalyst 5 is detected by means of the no x measurement device 10 as an no x actual value and transmitted to the control device 12 , in the control device 12 the no x actual value determined is compared with a predefined or calculated no x nominal value as an expected value . if this comparison reveals too great a deviation of the no x actual value from the no x nominal value , a defective or correspondingly deteriorated function capacity of the no oxidation catalyst 5 can be concluded . if the actual deviation found is too great , then , for example via a further control line 20 a fault signal can be emitted by the control device 12 . in principle it is also possible to arrange the no x measurement device 10 in the exhaust pipe 4 downstream after the scr catalyst 7 so that the scr catalyst 7 is included in the test of function capacity . with such an arrangement of the no x measurement device 10 , during testing of the function capacity of the no oxidation catalyst 5 , at the point marked with arrow 9 no reducing agent is added for the selective catalytic reduction . instead of generating larger quantities of reducing agent upstream in the exhaust gas flow before the no oxidation catalyst 5 by a change in operating parameters , a targeted addition of hydrocarbons as a reducing agent can take place into the exhaust gas pipe 4 before the no oxidation catalyst 5 . at the end of the test phase in which the function capacity of the no oxidation catalyst 5 is tested , the internal combustion engine 1 is returned to normal operation . the test phases can then be carried out regularly at greater time intervals . the flow diagram in fig2 shows the individual method steps which are performed on testing the function capacity of the no oxidation catalyst 5 . in the method shown , after the method start it is first checked whether the engine is in a stationary operation . if so , in the next method step the no x concentration downstream after the no oxidation catalyst 5 ( abbreviated to no oxi - cat ) is determined . then the no x concentration is stored as a first actual value no x , 0 . then the co and / or hc concentration upstream before the no oxidation catalyst 5 is increased in the manner described above , in particular by the adjustment of operating parameters of the internal combustion engine 1 . by means of the no x measurement device 10 in fig1 , the no x concentration n x , 1 downstream after the no oxidation catalyst 5 is then detected as a second actual value . this second actual value of no x concentration can be compared by subtraction with the previously stored value n x , 0 . for the case where the difference determined between the two actual values is less than a predefined expected value , a fault message is output which indicates a reduced or defective function of the no oxidation catalyst 5 . if the difference determined is however greater than or equal to a predefined nominal or expected value , the test phase is exited again and no fault message triggered . for the case that the no x measurement device 10 provided in fig1 is arranged in the exhaust gas flow after the scr catalyst 7 , before determining the two actual values the addition of urea or a corresponding reducing agent provided for the selective catalytic reduction is stopped .