Control circuit for qualifying a fault for a synchronous machine, comprising: a rotor position angle range determination device (12; 112) which is designed to determine at least one rotor position angle range which is covered by a rotor of the synchronous machine; a diagnostic signal determination device (14; 114) which is designed to determine at least one diagnostic signal value of a diagnostic signal which indicates a fault; a comparison device which is designed to determine whether a predetermined diagnostic threshold is exceeded by the at least one determined diagnostic signal value or a diagnostic signal value which is derived from the at least one diagnostic signal value; and a qualification device which is designed to qualify the fault, which is indicated by the diagnostic signal, on the basis of the at least one covered rotor position angle range and the situation of the predetermined diagnostic threshold being exceeded by the diagnostic signal value or the derived diagnostic signal value; wherein the diagnostic signal determination device is designed to continuously determine the diagnostic signal value while the rotor covers the at least one rotor position angle range; wherein the control circuit has an integration device which is designed to calculate an integral over a covered rotor position angle within the at least one rotor position angle range with a constant as integrand for as long as the continuously determined diagnostic signal value exceeds the predetermined diagnostic threshold; and wherein the qualification device is designed to qualify the fault if the integral exceeds a predetermined value. a rotor position angle range determination device (12; 112) which is designed to determine at least one rotor position angle range which is covered by a rotor of the synchronous machine; a diagnostic signal determination device (14; 114) which is designed to determine at least one diagnostic signal value of a diagnostic signal which indicates a fault; a comparison device which is designed to determine whether a predetermined diagnostic threshold is exceeded by the at least one determined diagnostic signal value or a diagnostic signal value which is derived from the at least one diagnostic signal value; and a qualification device which is designed to qualify the fault, which is indicated by the diagnostic signal, on the basis of the at least one covered rotor position angle range and the situation of the predetermined diagnostic threshold being exceeded by the diagnostic signal value or the derived diagnostic signal value; wherein the diagnostic signal determination device is designed to continuously determine the diagnostic signal value while the rotor covers the at least one rotor position angle range; wherein the control circuit has an integration device which is designed to calculate an integral over a covered rotor position angle within the at least one rotor position angle range with a constant as integrand for as long as the continuously determined diagnostic signal value exceeds the predetermined diagnostic threshold; and wherein the qualification device is designed to qualify the fault if the integral exceeds a predetermined value.