Improved sub-assemblies and methods of control for use in a diagnostic assay system adapted to receive an assay cartridge are provided herein. Such sub-assemblies include: a brushless DC motor, a door opening/closing mechanism and cartridge loading mechanism, a syringe and valve drive mechanism assembly, a sonication horn, a thermal control device and optical detection/excitation device. Such systems can further include a communications unit configured to wirelessly communicate with a mobile device of a user so as to receive a user input relating to functionality of the system with respect to an assay cartridge received therein and relaying a diagnostic result relating to the assay cartridge to the mobile device.