A method for detecting errors in individual color separation images of a multi-color printing machine, in particular an electrophotographic printing machine, comprising a plurality of printing units, is described. Using this method, first a plurality of first register lines is printed with a first printing unit, and a plurality of second register lines is printed with a second printing unit in such a manner that each of the first register lines, together with one of said second register lines, is positioned inside a respective registration frame. Then the plurality of the first and second register lines in the respective registration frame are detected with a register sensor, and an output signal of the register sensor relating to the respective second register line is compared with an intensity threshold value in order to determine whether the second register lines can be recognized. In an alternative embodiment of the method, a plurality of the first register lines is printed with a first printing unit in such a manner that each of the first register lines is printed within a respective registration frame. Subsequently, the plurality of the first register lines in the respective registration frames is detected with a register sensor, and an output signal of the register sensor relating to the respective first register lines is compared with a pre-specified intensity threshold value in order to determine whether the first register lines are recognizable.