High density polyethylene (HDPE) is widely used in automotive industry applications. When a specimen made of HDPE tested under cyclic loading, the inelastic deformation causes heat generated within the material, resulting in a temperature rise. The specimen temperature would stabilize if heat transfer from specimen surface can balance with the heat generated. Otherwise, the temperature will continue to rise, leading to a thermo assist failure. It is shown in this study that both frequencies and stress levels contribute to the temperature rise. Under service conditions, most of the automotive components experience low cyclic load frequency much less than 1 Hz. However, the frequency is usually set to a higher constant number for different stress levels in current standard fatigue life tests. This practice may lead to confusion in understanding the failure mechanism of polymer material and the fatigue data obtained from the lab test would not be appropriate for evaluation of the real components. In order to clarify this confusion, a critical stress-frequency failure map is proposed in this paper to identify if the failure is due to overheating or crack propagation. Additionally a mathematical methodology is developed to model temperature increase due to energy dissipation under cyclic loading, with help of which the critical stress-frequency failure map is numerically predicted. Good agreement is found between the experimental results and model predictions, which sheds light on thorough understanding of the complicated failure mechanism in thermoplastic polymers.