Spur gears can be made using a variety of materials, including steel, aluminium, and polymer. Polymer gears are usually employed because they have different advantages over steel gears, such as great corrosion resistance, significant weight reduction, the ability to be used with or without lubrication, and especially a lower manufacturing price. However, failure modes prevent the use of polymer gears in all industrial applications. High temperatures and loads can threaten the polymer gear’s life cycle by leading to the start of critical failure processes, such as cracks. Consequently, it is essential to examine how damage impacts the performance of polymer gears. Using the Extended Finite Element Method (XFEM) with a cohesive segment technique based on traction-separation laws, this study aims to examine the performance of a polyamide (PA12) spur gear. Specifically, this method will estimate the propagation of crack damage under different load magnitudes. In this study, a static analysis is conducted to evaluate the influence of various artificial fracture dimensions introduced into the tooth root on the mesh stiffness (MS), utilising a single-gear tooth model over one complete mesh cycle. Results show the influence of the different crack parameters on gear performance. A significant crack length and precise deviation lead to a reduction in the MS.