What does the APU MASTER switch FAULT light indicate when it illuminates amber?

The illumination of the APU MASTER switch FAULT light in amber signifies that there has been a fault detected in the Auxiliary Power Unit (APU), leading to an automatic shutdown of the APU. This is a crucial safety feature designed to protect the aircraft systems, as a fault could indicate potential problems such as over-temperature or over-speed conditions that could jeopardize the integrity of the APU's operation. The automatic shutdown helps prevent further damage and ensures that the aircraft systems remain safe for operation.

When the APU MASTER switch FAULT light is illuminated, it is imperative for crew members to be aware that the APU is no longer operational and to take necessary actions in accordance with the aircraft's standard operating procedures. The other choices do not represent conditions that would cause the amber light to illuminate, thus reinforcing the significance of the fault indication as a trigger for immediate attention and corrective measures.