What is the most likely direct cause for a plenum limit control to actuate?

A plenum limit control is designed to monitor the temperature in the plenum area of a heating or air conditioning system. Its primary function is to prevent overheating by shutting down the system if the temperature exceeds a predetermined limit. When the blower fails to function correctly, it directly affects the circulation of air throughout the system. This lack of airflow can lead to a rapid increase in temperature within the plenum, triggering the limit control to actuate in an effort to protect the system from potential damage.

In contrast, while external temperature changes, thermostat malfunctions, or excessive humidity can influence the overall temperature regulation and comfort inside a space, they do not directly cause the limit control to actuate in the way that a malfunctioning blower does. The blower's primary role is to ensure proper air movement; without it, the system overheats more quickly, leading to the plenum limit control engaging as a safety measure.