Which fault light would indicate a complete loss of AUX power?

The CRITICAL DEFECT fault light indicates a complete loss of AUX power because this light is designed to signal serious issues that can affect the overall functionality of the system. AUX power typically supports various ancillary systems and control functions necessary for the proper operation of train systems, and its failure could lead to significant operational risks or safety concerns. The illumination of the CRITICAL DEFECT light thereby alerts operators to a fundamental problem that compromises the system's performance, necessitating immediate attention.

In contrast, other fault lights serve different purposes. For example, the SURGE RES fault light may indicate issues related to voltage or current surges, while the REDUCED PERFORMANCE light points to operational limitations without necessarily indicating total power loss. The SAFETY LOOP fault light concerns issues within safety monitoring loops specifically, rather than a broad AUX power failure. Each of these other lights conveys specific fault conditions rather than the overarching critical nature of a complete AUX power loss.