Method to operate an air jet spinning machine, - in which the air jet spinning machine has at least one spinning position with a spinning nozzle for manufacturing a yarn, - in which a fiber strand is fed to the spinning nozzle through an inlet during the operation of the spinning position, - in which the fiber strand is imparted a twist inside a vortex chamber of the spinning nozzle by means of a swirled air current, so that a yarn is formed from the fiber strand that finally leaves the spinning nozzle through an outlet, and - in which an additive is added with the help of an additive dispenser, at least temporarily, to the spinning position while the air jet spinning machine is operating and applied on the fiber strand and/or the yarn or on parts of the spinning nozzle, - in which the yarn leaving the outlet is monitored concerning at least one physical parameter with the help of a sensor system, characterized in that - based on at least one measured value supplied by the sensor system correlated with the above-mentioned parameter, it is determined whether and/or how much additive was applied on the fiber strand or the yarn manufactured from this that passed through the sensor system, - in which in which the yarn is also monitored with the aid of the sensor system to determine whether the thickness and/or mass of the yarn exceeds or not reach predetermined limit values in a defined manner, - in which the sensor system is connected to a control unit of the air spinning machine, - in which the control unit interrupts the manufacturing of the yarn when at least one of the limits is exceeded or not reached in a defined way, and - in which the mass and/or volumetric flow of the additive supplied during a cleaning operation of the spinning position is higher than during normal operation, in which case at least one of the limits has another value during the cleaning operation than during normal operation. - in which the air jet spinning machine has at least one spinning position with a spinning nozzle for manufacturing a yarn, - in which a fiber strand is fed to the spinning nozzle through an inlet during the operation of the spinning position, - in which the fiber strand is imparted a twist inside a vortex chamber of the spinning nozzle by means of a swirled air current, so that a yarn is formed from the fiber strand that finally leaves the spinning nozzle through an outlet, and - in which an additive is added with the help of an additive dispenser, at least temporarily, to the spinning position while the air jet spinning machine is operating and applied on the fiber strand and/or the yarn or on parts of the spinning nozzle, - in which the yarn leaving the outlet is monitored concerning at least one physical parameter with the help of a sensor system, characterized in that - based on at least one measured value supplied by the sensor system correlated with the above-mentioned parameter, it is determined whether and/or how much additive was applied on the fiber strand or the yarn manufactured from this that passed through the sensor system, - in which in which the yarn is also monitored with the aid of the sensor system to determine whether the thickness and/or mass of the yarn exceeds or not reach predetermined limit values in a defined manner, - in which the sensor system is connected to a control unit of the air spinning machine, - in which the control unit interrupts the manufacturing of the yarn when at least one of the limits is exceeded or not reached in a defined way, and - in which the mass and/or volumetric flow of the additive supplied during a cleaning operation of the spinning position is higher than during normal operation, in which case at least one of the limits has another value during the cleaning operation than during normal operation.