What happens when the air temperature in the duct exceeds safe levels?

When the air temperature in the duct exceeds safe levels, the DUCT TEMP light illuminates. This indicator serves as a warning to the pilot that the temperature is beyond the acceptable operating range. The illumination of the DUCT TEMP light prompts the crew to take necessary precautions to prevent overheating and potential damage to the system. It indicates a need for immediate attention to ensure the safety and functionality of the aircraft's environmental control system.

The other options, while related to aircraft safety and operational procedures, do not accurately describe the immediate consequence of exceeding duct temperature limits. The system shutting down may be a response to various issues, but the specific signal of duct temperature exceeding safe levels is the DUCT TEMP light coming on. Cabin pressure decreases is not directly linked to duct temperature; rather, it relates to pressurization systems. Emergency procedures could be initiated as a later response to an ongoing issue, but the first alert for this specific situation is the illumination of the DUCT TEMP light.