A technician starts up a standing pilot furnace, but the main burners light and the furnace cycles off after 30 seconds. What might be the issue if the blower did not activate?

The situation described indicates that the furnace is experiencing a shutdown shortly after the burners ignite, and importantly, the blower did not activate. The primary role of the fan switch is to control the operation of the blower; it typically signals the blower to turn on when heat is detected within the furnace. If the fan switch were faulty, it could cause the blower not to operate, leading to an overheating condition that would cause the furnace to cycle off, even if the burners initially lit.

Meanwhile, a defective thermocouple could also contribute to the problem. The thermocouple is a safety device that detects whether the pilot light is on. If it fails, it may not provide the necessary signal to keep the gas valve open, causing the burners to shut down prematurely.

Given the factors, if the blower is not working and the main burners cycle off after a short duration, malfunctioning components such as a faulty fan switch or a defective thermocouple could directly be related to the observed problem. Therefore, the correct answer includes either potential cause—indicating that both issues can lead to the same symptom of the furnace cycling off without proper blower operation.