Method for operating a metal detector that comprises a balanced coil system with a transmitter coil that is connected to a transmitter unit and with a first and a second receiver coil (22A, 22B) that are connected to an input of a receiver unit, which is connected to a signal processing unit, which transmitter unit comprises a transmitter signal path (tp) for which a transmitter signal (tx) with at least one fixed or selectable operating frequency is provided, which transmitter signal (tx) is applied to an input of a transmitter amplifier that forwards the amplified transmitter signal (tx) directly or via a transmitter matching unit to the transmitter coil; which transmitter unit provides a reference transmitter signal (tx) and a reference quadrature signal (tx90°) to the receiver unit or to the signal processing unit; which receiver unit comprises at least one receiver signal path (rp) in which the receiver signal (rs) received from the balanced coil system is applied directly or via a receiver matching unit to a receiver amplifier, which forwards the amplified receiver signal (rs) directly or indirectly to a receiver demodulator (35; 4535) that is provided in the receiver unit or is implemented in the signal processing unit; which receiver demodulator (35; 4535), based on the reference transmitter signal (tx) and the reference quadrature signal (tx90°) received from the transmitter unit, provides a demodulated complex receiver signal (rsc) with in-phase receiver signal components (rs-I) and quadrature receiver signal components (rs-Q); which in-phase receiver signal components (rs-I) and quadrature receiver signal components (rs-Q) are processed in least one signal processing path (sp) provided in the signal processing unit, in which signal components of the complex receiver signal (rsc) that relate to products or noise are suppressed and in which signal components originating from metal contaminants are further processed, characterised by the steps of providing at least one measurement signal (ms) taken from the receiver signal path (rp) to a measurement channel; analysing the measurement signal (ms) in an evaluation module or in a receiver control module to provide related measurement information, and, based on the obtained measurement information; determining in the receiver control module if a critical condition, in which signal processing in the receiver signal path (rp) is possibly impaired, is present; and providing a procedure for handling critical conditions in the metal detector that is executed if a critical condition is present. which transmitter unit comprises a transmitter signal path (tp) for which a transmitter signal (tx) with at least one fixed or selectable operating frequency is provided, which transmitter signal (tx) is applied to an input of a transmitter amplifier that forwards the amplified transmitter signal (tx) directly or via a transmitter matching unit to the transmitter coil; which transmitter unit provides a reference transmitter signal (tx) and a reference quadrature signal (tx90°) to the receiver unit or to the signal processing unit; which receiver unit comprises at least one receiver signal path (rp) in which the receiver signal (rs) received from the balanced coil system is applied directly or via a receiver matching unit to a receiver amplifier, which forwards the amplified receiver signal (rs) directly or indirectly to a receiver demodulator (35; 4535) that is provided in the receiver unit or is implemented in the signal processing unit; which receiver demodulator (35; 4535), based on the reference transmitter signal (tx) and the reference quadrature signal (tx90°) received from the transmitter unit, provides a demodulated complex receiver signal (rsc) with in-phase receiver signal components (rs-I) and quadrature receiver signal components (rs-Q); which in-phase receiver signal components (rs-I) and quadrature receiver signal components (rs-Q) are processed in least one signal processing path (sp) provided in the signal processing unit, in which signal components of the complex receiver signal (rsc) that relate to products or noise are suppressed and in which signal components originating from metal contaminants are further processed, characterised by the steps of providing at least one measurement signal (ms) taken from the receiver signal path (rp) to a measurement channel; analysing the measurement signal (ms) in an evaluation module or in a receiver control module to provide related measurement information, and, based on the obtained measurement information; determining in the receiver control module if a critical condition, in which signal processing in the receiver signal path (rp) is possibly impaired, is present; and providing a procedure for handling critical conditions in the metal detector that is executed if a critical condition is present.