Applications of short fibre reinforced polymer composites (SFRPCs) have been rapidly increasing and most of the components made of these materials are subjected to cyclic loading. In automotive applications, “under the hood” is the harshest environmental condition for plastic-based materials with temperatures ranging from -40°C to 120°C. Components are subjected to mechanical vibrations primarily as a result of the periodic excitation and the dynamics of the engine firing. It is important, therefore, to design and test the components accurately so as to minimise the risk of component failure during the expected lifetime of the vehicle. Taking this into account, this thesis investigated if the current test methods being used at Scania ensured a valid fatigue testing of engine components made of SFRPCs. An extensive literature review was carried out detailing the work published on SFRPC fatigue to-date and the methods currently used at Scania NMBT were detailed. A series of sine and random vibration tests were then performed to characterise material behaviour in addition to Dynamic Mechanical Analysis and Scanning Electron Microscopy of failed specimens. The results of these, combined with the knowledge gathered in the literature review, resulted in a number of suggestions to adapt the current test methods with the aim of increasing their validity for SFRPCs.