The article concerns the analysis of the impact of temperature changes (specifically temperature reduction) on the operating conditions and reliability of an external gear pump. One of the hydraulic pumps manufactured in the production plant was tested to determine the conditions necessary for the seizure of its components. Measurements were taken at an ambient temperature of −22 °C and an oil temperature in the tank of 55 °C. An analysis of external gear pump designs from various manufacturers reveals that pumps with cast iron bodies are intended for more demanding operating conditions. This is primarily due to the superior mechanical properties of cast iron compared to aluminum alloys. When considering low-temperature operating conditions and the associated phenomena and potential risks, it becomes clear that gear pumps incorporating cast iron components perform better. This advantage is not only due to mechanical characteristics like hardness and scratch resistance, but also to the significant difference in the thermal expansion coefficients between the two materials. As demonstrated, aluminum alloys exhibit more than twice the thermal expansion of cast iron under the same conditions. Therefore, when selecting components for a hydraulic system, it is essential to understand the environmental and operational conditions the system will face. This knowledge is critical to accurately predicting potential issues and selecting appropriate components to ensure reliable, failure-free operation. The results showed that due to the increased values of internal resistance during start-up in thermal shock conditions resulting from the different dynamics of heating of the pump components and differences in thermal expansion of these components, the hydraulic-mechanical efficiency, and consequently also the mechanical efficiency, is initially lower than the nominal values of an efficient pump.