Abstract: A method for detecting a blocked valve of a coolant compressor including a drive unit and a piston-cylinder unit for cyclical compression of a coolant, wherein the drive unit has an electric motor for driving the piston-cylinder unit, monitors the speed (?) of the electric motor. A maximum speed (?max) of the electric motor is initially detected, and the following steps are carried out, as long as the speed (?) of the electric motor substantially corresponds to the maximum speed (?max): determining a maximum value Xmax of a monitoring parameter (I, T) of the coolant compressor (1); determining a value Xt1 of the monitoring parameter (I, T) after a first time period (t1) after determining the maximum value Xmax; detecting a blocked valve if Xt1 is less than Xmax and (Xmax?Xt1)/Xmax??X applies, wherein ?X is predetermined.