The failures in reducing speed gearboxes or speed multipliers with gears have traditionally been diagnosed by means of vibration analysis. Recently there has been noticed the possibility of making a diagnosis based on the tracking of the stator current of the motor driving the box or its electromagnetic torque. These variations, unlike vibration studies, have the advantage of being non-invasive. This work aims to evaluate the ability of fault detection and diagnosis from the spectral analysis of the torque. On an actual application, the behavior of the frequencies associated with the fault according to the engine load condition is analyzed. Conclusions about the possibility of detecting failures of different severity over a tooth of a toothed wheel are obtained.