A method and a device for operating an internal combustion engine are discussed in DE 10 2005 051 701 A1, in which an overall injection is subdivided into a basic injection and at least one measured injection. The injection period of the measured injection is successively reduced and the injection period of the basic injection is increased, in such a way that an overall injection quantity determined from a characteristics curve of the valve remains unchanged. A deviation of a variable characterizing an actual mixture, provoked by the successive reduction of the injection period of the measured injection, from a variable characterizing a setpoint mixture is detected. The deviation or a characteristics curve of the fuel injector is adapted or corrected. The detection as to whether the actual fuel-air mixture is deviating from the setpoint fuel-air mixture take place with the aid of a Lambda value provided by a Lambda sensor.