Faults detection in planet gears of planetary gearboxes by vibration analysis is not as straightforward as the detection of failures in conventional gear transmissions. This is because the driving flanks are different between the inner contact (ring-planet gear) and the external contact (sun-planet gear). Furthermore, the failure position varies with respect to the fixed transducer located on the ring gear, a situation leading to differences in the transient amplitude of the acceleration signals. In the present study, the planet gear fault is simulated based on two models: dynamic and phenomenological. These models have been used in other publications for the study of non-faulty planetary gearboxes. For the first model, the equations of motion are solved by temporal integration using HHT; whereas the second model is solved using algebraic equations. The fundamental results from the simulation of both models are shown in terms of waveforms and spectral content. They are also compared and validated later with the experimental results extracted from two test benches. It is concluded that the dynamic model has larger difficulties for spectral analysis, due to numerical errors from the temporal integration and mainly by the non-inertial reference frame used. Regarding the detection of failures in planet gears through vibration signals, this will not necessarily generate periodic transients every half rotation of this component, as it depends on if the planet gear is damaged on one or both flanks of the tooth. The conducted analysis also permits to conclude that when one flank is damaged, it can be detected more clearly when it meshes with the sun gear than when it meshes with the ring gear.