In a two-channel magnetic metal detector having first and second mutually inductively coupled detection coils connected to first and second detection channels, respectively, the inductance of the coils is tested by applying a square wave test signal to the first detection coil and determining if the resulting output signal from the second detection channel has a magnitude falling within a range of magnitudes defined by two threshold values stored in a non-volatile memory. A microcomputer applies digital signals to a digital to analog converter to generate the square wave. The output signal from the second detection channel is repetitively sampled and digitized and the digital values are transferred to the microcomputer which includes means for comparing the digital values with the threshold values. If a comparison determines that the magnitude of the output signal from the second detection channel does not fall within the range of magnitudes defined by the threshold values, the microcomputer generates an error indication.