evaluation is possible an operational step 21 selects the cylinder whose ignition diagnostic signal zund ok is to be weighted in the following operational step 22 the ignition diagnostic signal zund ok of this cylinder z is now used for evaluation after each point of ignition z an interrogation 23 checks whether the ignition diagnostic signal zund ok is equal to zero if this is the case i e if the ignition in the cylinder was in order the value 1 is subtracted from the weighted ignition diagnostic value zuntab in an operational step 24 an interrogation 25 then checks whether zuntab lt 0 if this is the case the operational step 26 resets the weighted ignition diagnostic value zuntab for this cylinder to zero the negative output of the interrogation 25 and the operational step 26 lead to the interrogation 27 if the interrogation 23 was answered with no i e if the ignition diagnostic signal for this cylinder was not correct then the weighted ignition diagnostic value zuntab is increased by an amount dafdzu this amount dafdzu is determined in practice for each engine type after this operational step 28 the interrogation 29 checks whether the weighted ignition diagnostic value zuntab has exceeded a maximum permissible limiting value afxdzu if this is the case the operational step 30 limits the weighted ignition diagnostic value to this maximum permissible value afxdzu the negative output of the interrogation 29 and the operational step 30 likewise lead to the interrogation 27 the interrogation 27 now checks whether the weighted ignition diagnostic value zuntab is greater than a threshold afsdzu which if exceeded means that measures to protect the catalyst should be introduced this threshold.